diff --git a/3rd-party/domain-mapping.php b/3rd-party/domain-mapping.php
index 6079ac325908..f780033b98dd 100644
--- a/3rd-party/domain-mapping.php
+++ b/3rd-party/domain-mapping.php
@@ -1,5 +1,7 @@
 <?php
 
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
 /**
  * Class Jetpack_3rd_Party_Domain_Mapping
  *
@@ -39,7 +41,7 @@ private function __construct() {
 	 * to try and hook a domain mapping plugin to the Jetpack sync filters for the home_url and site_url callables.
 	 */
 	function attempt_to_hook_domain_mapping_plugins() {
-		if ( ! Jetpack_Constants::is_defined( 'SUNRISE' ) ) {
+		if ( ! Constants_Manager::is_defined( 'SUNRISE' ) ) {
 			return;
 		}
 
@@ -58,7 +60,7 @@ function attempt_to_hook_domain_mapping_plugins() {
 	 * @return bool
 	 */
 	function hook_wordpress_mu_domain_mapping() {
-		if ( ! Jetpack_Constants::is_defined( 'SUNRISE_LOADED' ) || ! $this->function_exists( 'domain_mapping_siteurl' ) ) {
+		if ( ! Constants_Manager::is_defined( 'SUNRISE_LOADED' ) || ! $this->function_exists( 'domain_mapping_siteurl' ) ) {
 			return false;
 		}
 
diff --git a/_inc/lib/core-api/wpcom-endpoints/subscribers.php b/_inc/lib/core-api/wpcom-endpoints/subscribers.php
index c1a712bdb84d..9f8d99994572 100644
--- a/_inc/lib/core-api/wpcom-endpoints/subscribers.php
+++ b/_inc/lib/core-api/wpcom-endpoints/subscribers.php
@@ -1,5 +1,7 @@
 <?php
 
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
 /**
  * Subscribers: Get subscriber count
  *
@@ -41,7 +43,7 @@ public function readable_permission_check() {
 	 */
 	public function get_subscriber_count( $request ) {
 		// Get the most up to date subscriber count when request is not a test
-		if ( ! Jetpack_Constants::is_defined( 'TESTING_IN_JETPACK' ) ) {
+		if ( ! Constants_Manager::is_defined( 'TESTING_IN_JETPACK' ) ) {
 			delete_transient( 'wpcom_subscribers_total' );
 		}
 
@@ -56,7 +58,7 @@ public function get_subscriber_count( $request ) {
 
 if (
 	Jetpack::is_module_active( 'subscriptions' ) ||
-	( Jetpack_Constants::is_defined( 'TESTING_IN_JETPACK' ) && Jetpack_Constants::get_constant( 'TESTING_IN_JETPACK' ) )
+	( Constants_Manager::is_defined( 'TESTING_IN_JETPACK' ) && Constants_Manager::get_constant( 'TESTING_IN_JETPACK' ) )
 ) {
 	wpcom_rest_api_v2_load_plugin( 'WPCOM_REST_API_V2_Endpoint_Subscribers' );
 }
diff --git a/_inc/lib/debugger/class-jetpack-debug-data.php b/_inc/lib/debugger/class-jetpack-debug-data.php
index 50642de30eb9..0cb10bdc6d37 100644
--- a/_inc/lib/debugger/class-jetpack-debug-data.php
+++ b/_inc/lib/debugger/class-jetpack-debug-data.php
@@ -5,6 +5,8 @@
  * @package jetpack
  */
 
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
 /**
  * Class Jetpack_Debug_Data
  *
@@ -323,8 +325,8 @@ public static function debug_data() {
 		$idc_urls = array(
 			'home'       => Jetpack_Sync_Functions::home_url(),
 			'siteurl'    => Jetpack_Sync_Functions::site_url(),
-			'WP_HOME'    => Jetpack_Constants::is_defined( 'WP_HOME' ) ? Jetpack_Constants::get_constant( 'WP_HOME' ) : '',
-			'WP_SITEURL' => Jetpack_Constants::is_defined( 'WP_SITEURL' ) ? Jetpack_Constants::get_constant( 'WP_SITEURL' ) : '',
+			'WP_HOME'    => Constants_Manager::is_defined( 'WP_HOME' ) ? Constants_Manager::get_constant( 'WP_HOME' ) : '',
+			'WP_SITEURL' => Constants_Manager::is_defined( 'WP_SITEURL' ) ? Constants_Manager::get_constant( 'WP_SITEURL' ) : '',
 		);
 
 		$debug_info['idc_urls']         = array(
diff --git a/class.jetpack-data.php b/class.jetpack-data.php
index 2d762866fc51..53c48681a9a2 100644
--- a/class.jetpack-data.php
+++ b/class.jetpack-data.php
@@ -1,5 +1,7 @@
 <?php
 
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
 class Jetpack_Data {
 	/*
 	 * Used internally when we want to look for the Normal Blog Token
@@ -82,8 +84,8 @@ public static function get_access_token( $user_id = false, $token_key = false )
 				$possible_normal_tokens[] = $stored_blog_token;
 			}
 
-			$defined_tokens = Jetpack_Constants::is_defined( 'JETPACK_BLOG_TOKEN' )
-				? explode( ',', Jetpack_Constants::get_constant( 'JETPACK_BLOG_TOKEN' ) )
+			$defined_tokens = Constants_Manager::is_defined( 'JETPACK_BLOG_TOKEN' )
+				? explode( ',', Constants_Manager::get_constant( 'JETPACK_BLOG_TOKEN' ) )
 				: array();
 
 			foreach ( $defined_tokens as $defined_token ) {
diff --git a/class.jetpack-gutenberg.php b/class.jetpack-gutenberg.php
index 70e9a51c5221..0175f27dd301 100644
--- a/class.jetpack-gutenberg.php
+++ b/class.jetpack-gutenberg.php
@@ -6,6 +6,8 @@
  * @package Jetpack
  */
 
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
 /**
  * Wrapper function to safely register a gutenberg block type
  *
@@ -220,7 +222,7 @@ public static function init() {
 		 * @param boolean
 		 */
 		if ( apply_filters( 'jetpack_load_beta_blocks', false ) ) {
-			Jetpack_Constants::set_constant( 'JETPACK_BETA_BLOCKS', true );
+			Constants_Manager::set_constant( 'JETPACK_BETA_BLOCKS', true );
 		}
 
 		/**
@@ -318,7 +320,7 @@ public static function get_jetpack_gutenberg_extensions_whitelist() {
 
 		$preset_extensions = isset( $preset_extensions_manifest->production ) ? (array) $preset_extensions_manifest->production : array();
 
-		if ( Jetpack_Constants::is_true( 'JETPACK_BETA_BLOCKS' ) ) {
+		if ( Constants_Manager::is_true( 'JETPACK_BETA_BLOCKS' ) ) {
 			$beta_extensions = isset( $preset_extensions_manifest->beta ) ? (array) $preset_extensions_manifest->beta : array();
 			return array_unique( array_merge( $preset_extensions, $beta_extensions ) );
 		}
@@ -548,7 +550,7 @@ public static function enqueue_block_editor_assets() {
 		}
 
 		$rtl        = is_rtl() ? '.rtl' : '';
-		$beta       = Jetpack_Constants::is_true( 'JETPACK_BETA_BLOCKS' ) ? '-beta' : '';
+		$beta       = Constants_Manager::is_true( 'JETPACK_BETA_BLOCKS' ) ? '-beta' : '';
 		$blocks_dir = self::get_blocks_directory();
 
 		$editor_script = plugins_url( "{$blocks_dir}editor{$beta}.js", JETPACK__PLUGIN_FILE );
diff --git a/class.jetpack-options.php b/class.jetpack-options.php
index fcb7fbc0ab38..4cf6bf28b7b3 100644
--- a/class.jetpack-options.php
+++ b/class.jetpack-options.php
@@ -1,5 +1,6 @@
 <?php
 
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
 use \Automattic\Jetpack\Options\Manager as Options_Manager;
 
 if ( class_exists( '\Automattic\Jetpack\Options\Manager' ) ) :
@@ -480,7 +481,7 @@ static function get_raw_option( $name, $default = null ) {
 	 */
 	static function bypass_raw_option( $name ) {
 
-		if ( Jetpack_Constants::get_constant( 'JETPACK_DISABLE_RAW_OPTIONS' ) ) {
+		if ( Constants_Manager::get_constant( 'JETPACK_DISABLE_RAW_OPTIONS' ) ) {
 			return true;
 		}
 		/**
diff --git a/class.jetpack.php b/class.jetpack.php
index 401b47590efe..2f532a3776bc 100644
--- a/class.jetpack.php
+++ b/class.jetpack.php
@@ -1,5 +1,7 @@
 <?php
 
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
 /*
 Options:
 jetpack_options (array)
@@ -1700,7 +1702,7 @@ public static function is_development_version() {
 		 */
 		return (bool) apply_filters(
 			'jetpack_development_version',
-			! preg_match( '/^\d+(\.\d+)+$/', Jetpack_Constants::get_constant( 'JETPACK__VERSION' ) )
+			! preg_match( '/^\d+(\.\d+)+$/', Constants_Manager::get_constant( 'JETPACK__VERSION' ) )
 		);
 	}
 
@@ -6163,10 +6165,10 @@ public static function get_sync_error_idc_option( $response = array() ) {
 	 * @return bool
 	 */
 	public static function sync_idc_optin() {
-		if ( Jetpack_Constants::is_defined( 'JETPACK_SYNC_IDC_OPTIN' ) ) {
-			$default = Jetpack_Constants::get_constant( 'JETPACK_SYNC_IDC_OPTIN' );
+		if ( Constants_Manager::is_defined( 'JETPACK_SYNC_IDC_OPTIN' ) ) {
+			$default = Constants_Manager::get_constant( 'JETPACK_SYNC_IDC_OPTIN' );
 		} else {
-			$default = ! Jetpack_Constants::is_defined( 'SUNRISE' ) && ! is_multisite();
+			$default = ! Constants_Manager::is_defined( 'SUNRISE' ) && ! is_multisite();
 		}
 
 		/**
@@ -6959,7 +6961,7 @@ public static function is_function_in_backtrace( $names ) {
 	 * @return string The URL to the file
 	 */
 	public static function get_file_url_for_environment( $min_path, $non_min_path ) {
-		$path = ( Jetpack_Constants::is_defined( 'SCRIPT_DEBUG' ) && Jetpack_Constants::get_constant( 'SCRIPT_DEBUG' ) )
+		$path = ( Constants_Manager::is_defined( 'SCRIPT_DEBUG' ) && Constants_Manager::get_constant( 'SCRIPT_DEBUG' ) )
 			? $non_min_path
 			: $min_path;
 
diff --git a/composer.lock b/composer.lock
index 82f9ed388be9..0ead19d09d85 100644
--- a/composer.lock
+++ b/composer.lock
@@ -40,13 +40,13 @@
             "dist": {
                 "type": "path",
                 "url": "./packages/constants",
-                "reference": "b2cebb6eb5d4da90374e17c232168088f76c4c49"
+                "reference": "75e0a6ac5dc5e0af8d1de1ae1106260f1b12cc33"
             },
             "type": "library",
             "autoload": {
-                "classmap": [
-                    "/legacy"
-                ]
+                "psr-4": {
+                    "Automattic\\Jetpack\\Constants\\": "src/"
+                }
             },
             "license": [
                 "GPL-2.0-or-later"
diff --git a/json-endpoints/jetpack/class.jetpack-json-api-plugins-endpoint.php b/json-endpoints/jetpack/class.jetpack-json-api-plugins-endpoint.php
index 1df4fe667121..9966c2f8ad35 100644
--- a/json-endpoints/jetpack/class.jetpack-json-api-plugins-endpoint.php
+++ b/json-endpoints/jetpack/class.jetpack-json-api-plugins-endpoint.php
@@ -1,5 +1,7 @@
 <?php
 
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
 /**
  * Base class for working with plugins.
  */
@@ -205,12 +207,12 @@ protected function get_file_mod_capabilities() {
 			$reasons_can_not_modify_files['has_no_file_system_write_access'] =  __( 'The file permissions on this host prevent editing files.', 'jetpack' );
 		}
 
-		$disallow_file_mods = Jetpack_Constants::get_constant('DISALLOW_FILE_MODS' );
+		$disallow_file_mods = Constants_Manager::get_constant('DISALLOW_FILE_MODS' );
 		if ( $disallow_file_mods ) {
 			$reasons_can_not_modify_files['disallow_file_mods'] =  __( 'File modifications are explicitly disabled by a site administrator.', 'jetpack' );
 		}
 
-		$automatic_updater_disabled = Jetpack_Constants::get_constant( 'AUTOMATIC_UPDATER_DISABLED' );
+		$automatic_updater_disabled = Constants_Manager::get_constant( 'AUTOMATIC_UPDATER_DISABLED' );
 		if ( $automatic_updater_disabled ) {
 			$reasons_can_not_autoupdate['automatic_updater_disabled'] = __( 'Any autoupdates are explicitly disabled by a site administrator.', 'jetpack' );
 		}
diff --git a/json-endpoints/jetpack/class.jetpack-json-api-plugins-modify-endpoint.php b/json-endpoints/jetpack/class.jetpack-json-api-plugins-modify-endpoint.php
index 49cf43dcd98a..df5617f5dd27 100644
--- a/json-endpoints/jetpack/class.jetpack-json-api-plugins-modify-endpoint.php
+++ b/json-endpoints/jetpack/class.jetpack-json-api-plugins-modify-endpoint.php
@@ -1,4 +1,7 @@
 <?php
+
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
 new Jetpack_JSON_API_Plugins_Modify_Endpoint(
 	array(
 		'description'          => 'Activate/Deactivate a Plugin on your Jetpack Site, or set automatic updates',
@@ -291,7 +294,7 @@ protected function deactivate() {
 	protected function update() {
 		$query_args = $this->query_args();
 		if ( isset( $query_args['autoupdate'] ) && $query_args['autoupdate'] ) {
-			Jetpack_Constants::set_constant( 'JETPACK_PLUGIN_AUTOUPDATE', true );
+			Constants_Manager::set_constant( 'JETPACK_PLUGIN_AUTOUPDATE', true );
 		}
 		wp_clean_plugins_cache();
 		ob_start();
diff --git a/json-endpoints/jetpack/class.jetpack-json-api-user-create-endpoint.php b/json-endpoints/jetpack/class.jetpack-json-api-user-create-endpoint.php
index bd71249b2254..dfb1f7b4974c 100644
--- a/json-endpoints/jetpack/class.jetpack-json-api-user-create-endpoint.php
+++ b/json-endpoints/jetpack/class.jetpack-json-api-user-create-endpoint.php
@@ -1,5 +1,7 @@
 <?php
 
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
 class Jetpack_JSON_API_User_Create_Endpoint extends Jetpack_JSON_API_Endpoint {
 
 	protected $needed_capabilities = 'create_users';
@@ -34,7 +36,7 @@ function create_or_get_user() {
 
 		$query_args = $this->query_args();
 		if ( isset( $query_args['invite_accepted'] ) && $query_args['invite_accepted'] ) {
-			Jetpack_Constants::set_constant( 'JETPACK_INVITE_ACCEPTED', true );
+			Constants_Manager::set_constant( 'JETPACK_INVITE_ACCEPTED', true );
 		}
 
 		if ( ! $user ) {
diff --git a/modules/protect.php b/modules/protect.php
index 334439fc29fe..b3cdbb14ca39 100644
--- a/modules/protect.php
+++ b/modules/protect.php
@@ -12,6 +12,8 @@
  * Additional Search Queries: security, jetpack protect, secure, protection, botnet, brute force, protect, login, bot, password, passwords, strong passwords, strong password, wp-login.php,  protect admin
  */
 
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
 include_once JETPACK__PLUGIN_DIR . 'modules/protect/shared-functions.php';
 
 class Jetpack_Protect_Module {
@@ -438,7 +440,7 @@ function check_login_ability( $preauth = false ) {
 		/**
 		 * JETPACK_ALWAYS_PROTECT_LOGIN will always disable the login page, and use a page provided by Jetpack.
 		 */
-		if ( Jetpack_Constants::is_true( 'JETPACK_ALWAYS_PROTECT_LOGIN' ) ) {
+		if ( Constants_Manager::is_true( 'JETPACK_ALWAYS_PROTECT_LOGIN' ) ) {
 			$this->kill_login();
 		}
 
diff --git a/modules/search/class.jetpack-search-helpers.php b/modules/search/class.jetpack-search-helpers.php
index 27526b4c36d2..462f71dc2bfe 100644
--- a/modules/search/class.jetpack-search-helpers.php
+++ b/modules/search/class.jetpack-search-helpers.php
@@ -7,6 +7,8 @@
  * @since      5.8.0
  */
 
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
 /**
  * Various helper functions for reuse throughout the Jetpack Search code.
  *
@@ -659,8 +661,8 @@ public static function are_filters_by_widget_disabled() {
 	 */
 	public static function site_has_vip_index() {
 		$has_vip_index = (
-			Jetpack_Constants::is_defined( 'JETPACK_SEARCH_VIP_INDEX' ) &&
-			Jetpack_Constants::get_constant( 'JETPACK_SEARCH_VIP_INDEX' )
+			Constants_Manager::is_defined( 'JETPACK_SEARCH_VIP_INDEX' ) &&
+			Constants_Manager::get_constant( 'JETPACK_SEARCH_VIP_INDEX' )
 		);
 
 		/**
diff --git a/modules/shortcodes/crowdsignal.php b/modules/shortcodes/crowdsignal.php
index f043d001c456..14f84fde8ae0 100644
--- a/modules/shortcodes/crowdsignal.php
+++ b/modules/shortcodes/crowdsignal.php
@@ -18,6 +18,8 @@
  * @package Jetpack
  */
 
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
 // Keep compatibility with the PollDaddy plugin.
 if (
 	! class_exists( 'CrowdsignalShortcode' )
@@ -213,7 +215,7 @@ public function crowdsignal_shortcode( $atts ) {
 			);
 
 			$inline = ! in_the_loop()
-				&& ! Jetpack_Constants::is_defined( 'TESTING_IN_JETPACK' );
+				&& ! Constants_Manager::is_defined( 'TESTING_IN_JETPACK' );
 
 			$no_script       = false;
 			$infinite_scroll = false;
diff --git a/modules/theme-tools/featured-content.php b/modules/theme-tools/featured-content.php
index 1df0f069c67a..7a33ddbff82c 100644
--- a/modules/theme-tools/featured-content.php
+++ b/modules/theme-tools/featured-content.php
@@ -1,5 +1,7 @@
 <?php
 
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
 if ( ! class_exists( 'Featured_Content' ) && isset( $GLOBALS['pagenow'] ) && 'plugins.php' !== $GLOBALS['pagenow'] ) {
 
 	/**
@@ -714,7 +716,7 @@ function wpcom_rest_request_before_callbacks( $request ) {
 		return $request;
 	}
 
-	if ( Jetpack_Constants::is_true( 'IS_WPCOM' ) && Jetpack_Constants::is_true( 'REST_API_REQUEST' ) ) {
+	if ( Constants_Manager::is_true( 'IS_WPCOM' ) && Constants_Manager::is_true( 'REST_API_REQUEST' ) ) {
 		add_filter( 'rest_request_before_callbacks', 'wpcom_rest_request_before_callbacks');
 	}
 
diff --git a/packages/constants/README.md b/packages/constants/README.md
new file mode 100644
index 000000000000..fe35798da3bd
--- /dev/null
+++ b/packages/constants/README.md
@@ -0,0 +1,56 @@
+# Jetpack Constants
+
+A simple constant manager for Jetpack.
+
+Testing constants is hard. Once you define a constant in PHP, it's defined. Constants Manager is an abstraction layer so that unit tests can set constants for tests.
+
+### Usage
+
+Retrieve the value of a constant `CONSTANT_NAME` (returns `null` if it's not defined):
+
+```php
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
+$constant_value = Constants_Manager::get_constant( 'CONSTANT_NAME' );
+```
+
+Set the value of a constant `CONSTANT_NAME` to a particular value:
+
+```php
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
+$value = 'some value';
+Constants_Manager::set_constant( 'CONSTANT_NAME', $value );
+```
+
+Check whether a constant `CONSTANT_NAME` is defined:
+
+```php
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
+$defined = Constants_Manager::is_defined( 'CONSTANT_NAME' );
+```
+
+Check whether a constant `CONSTANT_NAME` is truthy:
+
+```php
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
+$is_truthy = Constants_Manager::is_true( 'CONSTANT_NAME' );
+```
+
+Delete the `CONSTANT_NAME` constant:
+
+```php
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
+Constants_Manager::clear_single_constant( 'CONSTANT_NAME' );
+```
+
+Delete all known constants:
+
+```php
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
+Constants_Manager::clear_constants();
+```
diff --git a/packages/constants/composer.json b/packages/constants/composer.json
index eeefcefdaeec..0c09306ea3be 100644
--- a/packages/constants/composer.json
+++ b/packages/constants/composer.json
@@ -4,9 +4,18 @@
 	"type": "library",
 	"license": "GPL-2.0-or-later",
 	"require": {},
+	"require-dev": {
+		"phpunit/phpunit": "^5.7 || ^6.5 || ^7.5"
+	},
 	"autoload": {
-		"classmap": [
-			"/legacy"
+		"psr-4": {
+			"Automattic\\Jetpack\\Constants\\": "src/"
+		}
+	},
+	"scripts": {
+		"phpunit": [
+			"@composer install",
+			"./vendor/phpunit/phpunit/phpunit --colors=always"
 		]
 	}
 }
diff --git a/packages/constants/legacy/class.jetpack-constants.php b/packages/constants/legacy/class.jetpack-constants.php
deleted file mode 100644
index 1c6390d331bf..000000000000
--- a/packages/constants/legacy/class.jetpack-constants.php
+++ /dev/null
@@ -1,94 +0,0 @@
-<?php
-
-/**
- * Class Jetpack_Constants
- *
- * Testing constants is hard. Once you define a constant, it's defined. Jetpack_Constants is an
- * abstraction layer so that unit tests can set "constants" for tests.
- *
- * To test your code, you'll need to swap out `defined( 'CONSTANT' )` with `Jetpack_Constants::is_defined( 'CONSTANT' )`
- * and replace `CONSTANT` with `Jetpack_Constants::get_constant( 'CONSTANT' )`. Then in the unit test, you can set the
- * constant with `Jetpack::set_constant( 'CONSTANT', $value )` and then clean up after each test with something like
- * this:
- *
- * function tearDown() {
- *     Jetpack_Constants::clear_constants();
- * }
- */
-class Jetpack_Constants {
-	static $set_constants = array();
-
-	/**
-	 * Checks if a "constant" has been set in Jetpack_Constants, and if not,
-	 * checks if the constant was defined with define( 'name', 'value ).
-	 *
-	 * @param $name string The name of the constant
-	 *
-	 * @return bool
-	 */
-	public static function is_defined( $name ) {
-		return array_key_exists( $name, self::$set_constants )
-			? true
-			: defined( $name );
-	}
-	/**
-	 * Checks if a "constant" has been set in Jetpack_Constants
-	 * and has the value of true
-	 *
-	 * @param $name string The name of the constant
-	 *
-	 * @return bool
-	 */
-	public static function is_true( $name ) {
-		return self::is_defined( $name) && self::get_constant( $name );
-	}
-
-	/**
-	 * Attempts to retrieve the "constant" from Jetpack_Constants, and if it hasn't been set,
-	 * then attempts to get the constant with the constant() function.
-	 *
-	 * @param $name
-	 *
-	 * @return mixed null if the constant does not exist or the value of the constant.
-	 */
-	public static function get_constant( $name ) {
-		if ( array_key_exists( $name,  self::$set_constants ) ) {
-			return self::$set_constants[ $name ];
-		}
-
-		return defined( $name ) ? constant( $name ) : null;
-	}
-
-	/**
-	 * Sets the value of the "constant" within Jetpack_Constants.
-	 *
-	 * @param $name  string The name of the "constant"
-	 * @param $value string The value of the "constant"
-	 */
-	public static function set_constant( $name, $value ) {
-		self::$set_constants[ $name ] = $value;
-	}
-
-	/**
-	 * Will unset a "constant" from Jetpack_Constants if the constant exists.
-	 *
-	 * @param $name string The name of the "constant"
-	 *
-	 * @return bool Whether the constant was removed.
-	 */
-	public static function clear_single_constant( $name ) {
-		if ( ! array_key_exists( $name, self::$set_constants ) ) {
-			return false;
-		}
-
-		unset( self::$set_constants[ $name ] );
-		return true;
-	}
-
-	/**
-	 * Resets all of the constants within Jetpack_Constants.
-	 */
-	public static function clear_constants() {
-		self::$set_constants = array();
-	}
-}
diff --git a/packages/constants/phpunit.xml.dist b/packages/constants/phpunit.xml.dist
new file mode 100644
index 000000000000..443d8a0ace77
--- /dev/null
+++ b/packages/constants/phpunit.xml.dist
@@ -0,0 +1,7 @@
+<phpunit bootstrap="tests/php/bootstrap.php" backupGlobals="false" colors="true">
+	<testsuites>
+		<testsuite name="main">
+			<directory prefix="test" suffix=".php">tests/php</directory>
+		</testsuite>
+	</testsuites>
+</phpunit>
diff --git a/packages/constants/src/Manager.php b/packages/constants/src/Manager.php
new file mode 100644
index 000000000000..ff3b03dfb90d
--- /dev/null
+++ b/packages/constants/src/Manager.php
@@ -0,0 +1,109 @@
+<?php
+/**
+ * A constants manager for Jetpack.
+ *
+ * @package jetpack-constants
+ */
+
+namespace Automattic\Jetpack\Constants;
+
+/**
+ * Class Automattic\Jetpack\Constants\Manager
+ *
+ * Testing constants is hard. Once you define a constant, it's defined. Constants Manager is an
+ * abstraction layer so that unit tests can set "constants" for tests.
+ *
+ * To test your code, you'll need to swap out `defined( 'CONSTANT' )` with `Automattic\Jetpack\Constants\Manager::is_defined( 'CONSTANT' )`
+ * and replace `CONSTANT` with `Automattic\Jetpack\Constants\Manager::get_constant( 'CONSTANT' )`. Then in the unit test, you can set the
+ * constant with `Automattic\Jetpack\Constants\Manager::set_constant( 'CONSTANT', $value )` and then clean up after each test with something like
+ * this:
+ *
+ * function tearDown() {
+ *     Automattic\Jetpack\Constants\Manager::clear_constants();
+ * }
+ */
+class Manager {
+	/**
+	 * A container for all defined constants.
+	 *
+	 * @access public
+	 * @static
+	 *
+	 * @var array.
+	 */
+	public static $set_constants = array();
+
+	/**
+	 * Checks if a "constant" has been set in constants Manager, and if not,
+	 * checks if the constant was defined with define( 'name', 'value ).
+	 *
+	 * @param string $name The name of the constant.
+	 *
+	 * @return bool
+	 */
+	public static function is_defined( $name ) {
+		return array_key_exists( $name, self::$set_constants )
+			? true
+			: defined( $name );
+	}
+	/**
+	 * Checks if a "constant" has been set in constants Manager
+	 * and has the value of true
+	 *
+	 * @param string $name The name of the constant.
+	 *
+	 * @return bool
+	 */
+	public static function is_true( $name ) {
+		return self::is_defined( $name ) && self::get_constant( $name );
+	}
+
+	/**
+	 * Attempts to retrieve the "constant" from constants Manager, and if it hasn't been set,
+	 * then attempts to get the constant with the constant() function.
+	 *
+	 * @param string $name The name of the constant.
+	 *
+	 * @return mixed null if the constant does not exist or the value of the constant.
+	 */
+	public static function get_constant( $name ) {
+		if ( array_key_exists( $name, self::$set_constants ) ) {
+			return self::$set_constants[ $name ];
+		}
+
+		return defined( $name ) ? constant( $name ) : null;
+	}
+
+	/**
+	 * Sets the value of the "constant" within constants Manager.
+	 *
+	 * @param string $name  The name of the constant.
+	 * @param string $value The value of the constant.
+	 */
+	public static function set_constant( $name, $value ) {
+		self::$set_constants[ $name ] = $value;
+	}
+
+	/**
+	 * Will unset a "constant" from constants Manager if the constant exists.
+	 *
+	 * @param string $name The name of the constant.
+	 *
+	 * @return bool Whether the constant was removed.
+	 */
+	public static function clear_single_constant( $name ) {
+		if ( ! array_key_exists( $name, self::$set_constants ) ) {
+			return false;
+		}
+
+		unset( self::$set_constants[ $name ] );
+		return true;
+	}
+
+	/**
+	 * Resets all of the constants within constants Manager.
+	 */
+	public static function clear_constants() {
+		self::$set_constants = array();
+	}
+}
diff --git a/packages/constants/tests/php/bootstrap.php b/packages/constants/tests/php/bootstrap.php
new file mode 100644
index 000000000000..48b34a7a581b
--- /dev/null
+++ b/packages/constants/tests/php/bootstrap.php
@@ -0,0 +1,3 @@
+<?php
+
+require_once __DIR__ . '/../../vendor/autoload.php';
diff --git a/packages/constants/tests/php/test_Manager.php b/packages/constants/tests/php/test_Manager.php
new file mode 100644
index 000000000000..9c52667c455d
--- /dev/null
+++ b/packages/constants/tests/php/test_Manager.php
@@ -0,0 +1,109 @@
+<?php
+
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+use PHPUnit\Framework\TestCase;
+
+class Test_Manager extends TestCase {
+	public function setUp() {
+		if ( ! defined( 'JETPACK__VERSION' ) ) {
+			define( 'JETPACK__VERSION', '7.5' );
+		}
+	}
+
+	public function tearDown() {
+		parent::tearDown();
+		Constants_Manager::$set_constants = array();
+	}
+
+	// Constants_Manager::is_defined()
+
+	function test_jetpack_constants_is_defined_when_constant_set_via_class() {
+		Constants_Manager::set_constant( 'TEST', 'hello' );
+		$this->assertTrue( Constants_Manager::is_defined( 'TEST' ) );
+	}
+
+	function test_jetpack_constants_is_defined_false_when_constant_not_set() {
+		$this->assertFalse( Constants_Manager::is_defined( 'UNDEFINED' ) );
+	}
+
+	function test_jetpack_constants_is_defined_true_when_set_with_define() {
+		$this->assertTrue( Constants_Manager::is_defined( 'JETPACK__VERSION' ) );
+	}
+
+	function test_jetpack_constants_is_defined_when_constant_set_to_null() {
+		Constants_Manager::set_constant( 'TEST', null );
+		$this->assertTrue( Constants_Manager::is_defined( 'TEST' ) );
+	}
+
+	// Constants_Manager::get_constant()
+
+	function test_jetpack_constants_default_to_constant() {
+		$this->assertEquals( Constants_Manager::get_constant( 'JETPACK__VERSION' ), JETPACK__VERSION );
+	}
+
+	function test_jetpack_constants_get_constant_null_when_not_set() {
+		$this->assertNull( Constants_Manager::get_constant( 'UNDEFINED' ) );
+	}
+
+	function test_jetpack_constants_can_override_previously_defined_constant() {
+		$test_version = '1.0.0';
+		Constants_Manager::set_constant( 'JETPACK__VERSION', $test_version );
+		$this->assertEquals( Constants_Manager::get_constant( 'JETPACK__VERSION' ), $test_version );
+	}
+
+	function test_jetpack_constants_override_to_null_gets_null() {
+		Constants_Manager::set_constant( 'JETPACK__VERSION', null );
+		$this->assertNull( Constants_Manager::get_constant( 'JETPACK__VERSION' ) );
+	}
+
+	// Constants_Manager::set_constant()
+
+	function test_jetpack_constants_set_constants_adds_to_set_constants_array() {
+		$key = 'TEST';
+		Constants_Manager::set_constant( $key, '1' );
+		$this->assertArrayHasKey( $key, Constants_Manager::$set_constants );
+		$this->assertEquals( '1', Constants_Manager::$set_constants[ $key ] );
+	}
+
+	// Constants_Manager::clear_constants()
+
+	function test_jetpack_constants_can_clear_all_constants() {
+		Constants_Manager::set_constant( 'JETPACK__VERSION', '1.0.0' );
+		Constants_Manager::clear_constants();
+		$this->assertEmpty( Constants_Manager::$set_constants );
+	}
+
+	// Constants_Manager::clear_single_constant()
+
+	function test_jetpack_constants_can_clear_single_constant() {
+		Constants_Manager::set_constant( 'FIRST', '1' );
+		Constants_Manager::set_constant( 'SECOND', '2' );
+
+		$this->assertCount( 2, Constants_Manager::$set_constants );
+
+		Constants_Manager::clear_single_constant( 'FIRST' );
+
+		$this->assertCount( 1, Constants_Manager::$set_constants );
+		$this->assertContains( 'SECOND', array_keys( Constants_Manager::$set_constants ) );
+	}
+
+	function test_jetpack_constants_can_clear_single_constant_when_null() {
+		Constants_Manager::set_constant( 'TEST', null );
+		$this->assertCount( 1, Constants_Manager::$set_constants );
+
+		Constants_Manager::clear_single_constant( 'TEST' );
+
+		$this->assertEmpty( Constants_Manager::$set_constants );
+	}
+
+	// Jetpack_Constant::is_true
+	function test_jetpack_constants_is_true_method() {
+		$this->assertFalse( Constants_Manager::is_true( 'FOO' ), 'unset constant returns true' );
+		Constants_Manager::set_constant( 'FOO', false );
+
+		$this->assertFalse( Constants_Manager::is_true( 'FOO' ), 'false constant returns true' );
+		Constants_Manager::set_constant( 'FOO', true );
+
+		$this->assertTrue( Constants_Manager::is_true( 'FOO' ), 'true constant returns false');
+	}
+}
diff --git a/packages/options/src/Manager.php b/packages/options/src/Manager.php
index 8f031fe83c5f..2723b1bd69b6 100644
--- a/packages/options/src/Manager.php
+++ b/packages/options/src/Manager.php
@@ -7,6 +7,8 @@
 
 namespace Automattic\Jetpack\Options;
 
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
 /**
  * The Jetpack Options Manager class that is used as a single gateway between WordPress options API
  * and Jetpack.
@@ -277,7 +279,7 @@ function get_raw_option( $name, $default = null ) {
 	 * @return bool
 	 */
 	function bypass_raw_option( $name ) {
-		if ( \Jetpack_Constants::get_constant( 'JETPACK_DISABLE_RAW_OPTIONS' ) ) {
+		if ( Constants_Manager::get_constant( 'JETPACK_DISABLE_RAW_OPTIONS' ) ) {
 			return true;
 		}
 		/**
diff --git a/sync/class.jetpack-sync-actions.php b/sync/class.jetpack-sync-actions.php
index 3f137d840e8d..3ed60732b0a3 100644
--- a/sync/class.jetpack-sync-actions.php
+++ b/sync/class.jetpack-sync-actions.php
@@ -1,5 +1,7 @@
 <?php
 
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
 require_once dirname( __FILE__ ) . '/class.jetpack-sync-modules.php';
 
 /**
@@ -78,7 +80,7 @@ static function add_sender_shutdown() {
 	}
 
 	static function should_initialize_sender() {
-		if ( Jetpack_Constants::is_true( 'DOING_CRON' ) ) {
+		if ( Constants_Manager::is_true( 'DOING_CRON' ) ) {
 			return self::sync_via_cron_allowed();
 		}
 
diff --git a/sync/class.jetpack-sync-functions.php b/sync/class.jetpack-sync-functions.php
index efae4bf89a4c..dc4a511ab08c 100644
--- a/sync/class.jetpack-sync-functions.php
+++ b/sync/class.jetpack-sync-functions.php
@@ -1,5 +1,7 @@
 <?php
 
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
 /*
  * Utility functions to generate data synced to wpcom
  */
@@ -193,8 +195,8 @@ public static function get_raw_or_filtered_url( $url_type ) {
 			: 'site_url';
 
 		if (
-			! Jetpack_Constants::is_defined( 'JETPACK_SYNC_USE_RAW_URL' ) ||
-			Jetpack_Constants::get_constant( 'JETPACK_SYNC_USE_RAW_URL' )
+			! Constants_Manager::is_defined( 'JETPACK_SYNC_USE_RAW_URL' ) ||
+			Constants_Manager::get_constant( 'JETPACK_SYNC_USE_RAW_URL' )
 		) {
 			$scheme = is_ssl() ? 'https' : 'http';
 			$url    = self::get_raw_url( $url_type );
@@ -269,8 +271,8 @@ public static function get_raw_url( $option_name ) {
 
 		// Since we disregard the constant for multisites in ms-default-filters.php,
 		// let's also use the db value if this is a multisite.
-		if ( ! is_multisite() && Jetpack_Constants::is_defined( $constant ) ) {
-			$value = Jetpack_Constants::get_constant( $constant );
+		if ( ! is_multisite() && Constants_Manager::is_defined( $constant ) ) {
+			$value = Constants_Manager::get_constant( $constant );
 		} else {
 			// Let's get the option from the database so that we can bypass filters. This will help
 			// ensure that we get more uniform values.
diff --git a/sync/class.jetpack-sync-module-plugins.php b/sync/class.jetpack-sync-module-plugins.php
index 876526abb6a5..fb5609d90066 100644
--- a/sync/class.jetpack-sync-module-plugins.php
+++ b/sync/class.jetpack-sync-module-plugins.php
@@ -1,5 +1,7 @@
 <?php
 
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
 class Jetpack_Sync_Module_Plugins extends Jetpack_Sync_Module {
 
 	private $action_handler;
@@ -65,7 +67,7 @@ public function on_upgrader_completion( $upgrader, $details ) {
 		switch ( $details['action'] ) {
 			case 'update':
 				$state  = array(
-					'is_autoupdate' => Jetpack_Constants::is_true( 'JETPACK_PLUGIN_AUTOUPDATE' ),
+					'is_autoupdate' => Constants_Manager::is_true( 'JETPACK_PLUGIN_AUTOUPDATE' ),
 				);
 				$errors = $this->get_errors( $upgrader->skin );
 				if ( $errors ) {
diff --git a/sync/class.jetpack-sync-module-posts.php b/sync/class.jetpack-sync-module-posts.php
index 0384b64ffe1d..6874ff47ae80 100644
--- a/sync/class.jetpack-sync-module-posts.php
+++ b/sync/class.jetpack-sync-module-posts.php
@@ -1,5 +1,7 @@
 <?php
 
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
 require_once dirname( __FILE__ ) . '/class.jetpack-sync-settings.php';
 
 class Jetpack_Sync_Module_Posts extends Jetpack_Sync_Module {
@@ -318,7 +320,7 @@ public function wp_insert_post( $post_ID, $post = null, $update = null ) {
 			false;
 
 		$state = array(
-			'is_auto_save'                 => (bool) Jetpack_Constants::get_constant( 'DOING_AUTOSAVE' ),
+			'is_auto_save'                 => (bool) Constants_Manager::get_constant( 'DOING_AUTOSAVE' ),
 			'previous_status'              => $previous_status,
 			'just_published'               => $just_published,
 			'is_gutenberg_meta_box_update' => $this->is_gutenberg_meta_box_update(),
diff --git a/sync/class.jetpack-sync-module-protect.php b/sync/class.jetpack-sync-module-protect.php
index a8dcd43e83ed..98e572ef37bd 100644
--- a/sync/class.jetpack-sync-module-protect.php
+++ b/sync/class.jetpack-sync-module-protect.php
@@ -1,5 +1,7 @@
 <?php
 
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
 /**
  * logs bruteprotect failed logins via sync
  */
@@ -16,7 +18,7 @@ function init_listeners( $callback ) {
 
 	function maybe_log_failed_login_attempt( $failed_attempt ) {
 		$protect = Jetpack_Protect_Module::instance();
-		if ( $protect->has_login_ability() && ! Jetpack_Constants::is_true( 'XMLRPC_REQUEST' ) ) {
+		if ( $protect->has_login_ability() && ! Constants_Manager::is_true( 'XMLRPC_REQUEST' ) ) {
 			do_action( 'jetpack_valid_failed_login_attempt', $failed_attempt );
 		}
 	}
diff --git a/sync/class.jetpack-sync-module-updates.php b/sync/class.jetpack-sync-module-updates.php
index 4da4414fc073..133007260ec3 100644
--- a/sync/class.jetpack-sync-module-updates.php
+++ b/sync/class.jetpack-sync-module-updates.php
@@ -1,5 +1,7 @@
 <?php
 
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
 class Jetpack_Sync_Module_Updates extends Jetpack_Sync_Module {
 
 	const UPDATES_CHECKSUM_OPTION_NAME = 'jetpack_updates_sync_checksum';
@@ -102,7 +104,7 @@ public function update_core( $new_wp_version ) {
 		// Core was autoudpated
 		if (
 			'update-core.php' !== $pagenow &&
-			! Jetpack_Constants::is_true( 'REST_API_REQUEST' ) // wp.com rest api calls should never be marked as a core autoupdate
+			! Constants_Manager::is_true( 'REST_API_REQUEST' ) // wp.com rest api calls should never be marked as a core autoupdate
 		) {
 			/**
 			 * Sync event that fires when core autoupdate was successful
diff --git a/sync/class.jetpack-sync-module-users.php b/sync/class.jetpack-sync-module-users.php
index f1cb47325c9b..f752cf40fc35 100644
--- a/sync/class.jetpack-sync-module-users.php
+++ b/sync/class.jetpack-sync-module-users.php
@@ -1,5 +1,7 @@
 <?php
 
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
 class Jetpack_Sync_Module_Users extends Jetpack_Sync_Module {
 	const MAX_INITIAL_SYNC_USERS = 100;
 
@@ -246,7 +248,7 @@ function user_register_handler( $user_id, $old_user_data = null ) {
 			return;
 		}
 
-		if ( Jetpack_Constants::is_true( 'JETPACK_INVITE_ACCEPTED' ) ) {
+		if ( Constants_Manager::is_true( 'JETPACK_INVITE_ACCEPTED' ) ) {
 			$this->add_flags( $user_id, array( 'invitation_accepted' => true ) );
 		}
 		/**
@@ -267,7 +269,7 @@ function add_user_to_blog_handler( $user_id, $old_user_data = null ) {
 			return;
 		}
 
-		if ( Jetpack_Constants::is_true( 'JETPACK_INVITE_ACCEPTED' ) ) {
+		if ( Constants_Manager::is_true( 'JETPACK_INVITE_ACCEPTED' ) ) {
 			$this->add_flags( $user_id, array( 'invitation_accepted' => true ) );
 		}
 		/**
diff --git a/sync/class.jetpack-sync-sender.php b/sync/class.jetpack-sync-sender.php
index 21f914819310..76748bfc85e4 100644
--- a/sync/class.jetpack-sync-sender.php
+++ b/sync/class.jetpack-sync-sender.php
@@ -1,5 +1,7 @@
 <?php
 
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
 require_once dirname( __FILE__ ) . '/class.jetpack-sync-queue.php';
 require_once dirname( __FILE__ ) . '/class.jetpack-sync-defaults.php';
 require_once dirname( __FILE__ ) . '/class.jetpack-sync-json-deflate-array-codec.php';
@@ -56,7 +58,7 @@ private function init() {
 	public function maybe_set_user_from_token() {
 		$jetpack       = Jetpack::init();
 		$verified_user = $jetpack->verify_xml_rpc_signature();
-		if ( Jetpack_Constants::is_true( 'XMLRPC_REQUEST' ) &&
+		if ( Constants_Manager::is_true( 'XMLRPC_REQUEST' ) &&
 			! is_wp_error( $verified_user )
 			&& $verified_user
 		) {
@@ -207,7 +209,7 @@ public function do_sync_for_queue( $queue ) {
 		}
 
 		/* Don't make the request block till we finish, if possible. */
-		if ( Jetpack_Constants::is_true( 'REST_REQUEST' ) || Jetpack_Constants::is_true('XMLRPC_REQUEST' ) ) {
+		if ( Constants_Manager::is_true( 'REST_REQUEST' ) || Constants_Manager::is_true('XMLRPC_REQUEST' ) ) {
 			$this->fastcgi_finish_request();
 		}
 
diff --git a/tests/php/3rd-party/test_class.jetpack-domain-mapping.php b/tests/php/3rd-party/test_class.jetpack-domain-mapping.php
index 3e53b86331ba..70aa5efe0ee1 100644
--- a/tests/php/3rd-party/test_class.jetpack-domain-mapping.php
+++ b/tests/php/3rd-party/test_class.jetpack-domain-mapping.php
@@ -1,5 +1,7 @@
 <?php
 
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
 require_once JETPACK__PLUGIN_DIR . '3rd-party/domain-mapping.php';
 
 // Extend with a public constructor so we can test
@@ -10,7 +12,7 @@ public function __construct() {
 
 class WP_Test_Domain_Mapping extends WP_UnitTestCase {
 	function tearDown() {
-		Jetpack_Constants::clear_constants();
+		Constants_Manager::clear_constants();
 		foreach ( $this->get_jetpack_sync_filters() as $filter ) {
 			remove_all_filters( $filter );
 		}
@@ -35,7 +37,7 @@ function test_domain_mapping_should_not_try_to_hook_when_sunrise_disable() {
 	}
 
 	function test_domain_mapping_should_stop_search_after_hooking_once() {
-		Jetpack_Constants::set_constant( 'SUNRISE', true );
+		Constants_Manager::set_constant( 'SUNRISE', true );
 
 		$stub = $this->getMockBuilder( 'MockDomainMapping' )
 			->setMethods( array( 'hook_wordpress_mu_domain_mapping', 'hook_wpmu_dev_domain_mapping' ) )
@@ -55,7 +57,7 @@ function test_domain_mapping_should_stop_search_after_hooking_once() {
 	}
 
 	function test_domain_mapping_mu_domain_mapping_not_hooked_when_function_not_exists() {
-		Jetpack_Constants::set_constant( 'SUNRISE_LOADED', true );
+		Constants_Manager::set_constant( 'SUNRISE_LOADED', true );
 
 		$stub = $this->getMockBuilder( 'MockDomainMapping' )
 			->setMethods( array( 'function_exists' ) )
@@ -74,7 +76,7 @@ function test_domain_mapping_mu_domain_mapping_not_hooked_when_function_not_exis
 	}
 
 	function test_domain_mapping_mu_domain_mapping_hooked_when_function_exists() {
-		Jetpack_Constants::set_constant( 'SUNRISE_LOADED', true );
+		Constants_Manager::set_constant( 'SUNRISE_LOADED', true );
 
 		$stub = $this->getMockBuilder( 'MockDomainMapping' )
 			->setMethods( array( 'function_exists' ) )
diff --git a/tests/php/general/test-class.jetpack-constants.php b/tests/php/general/test-class.jetpack-constants.php
deleted file mode 100644
index ed626a7c8d8b..000000000000
--- a/tests/php/general/test-class.jetpack-constants.php
+++ /dev/null
@@ -1,100 +0,0 @@
-<?php
-
-class WP_Test_Jetpack_Constants extends WP_UnitTestCase {
-	public function tearDown() {
-		parent::tearDown();
-		Jetpack_Constants::$set_constants = array();
-	}
-
-	// Jetpack_Constants::is_defined()
-
-	function test_jetpack_constants_is_defined_when_constant_set_via_class() {
-		Jetpack_Constants::set_constant( 'TEST', 'hello' );
-		$this->assertTrue( Jetpack_Constants::is_defined( 'TEST' ) );
-	}
-
-	function test_jetpack_constants_is_defined_false_when_constant_not_set() {
-		$this->assertFalse( Jetpack_Constants::is_defined( 'UNDEFINED' ) );
-	}
-
-	function test_jetpack_constants_is_defined_true_when_set_with_define() {
-		$this->assertTrue( Jetpack_Constants::is_defined( 'JETPACK__VERSION' ) );
-	}
-
-	function test_jetpack_constants_is_defined_when_constant_set_to_null() {
-		Jetpack_Constants::set_constant( 'TEST', null );
-		$this->assertTrue( Jetpack_Constants::is_defined( 'TEST' ) );
-	}
-
-	// Jetpack_Constants::get_constant()
-
-	function test_jetpack_constants_default_to_constant() {
-		$this->assertEquals( Jetpack_Constants::get_constant( 'JETPACK__VERSION' ), JETPACK__VERSION );
-	}
-
-	function test_jetpack_constants_get_constant_null_when_not_set() {
-		$this->assertNull( Jetpack_Constants::get_constant( 'UNDEFINED' ) );
-	}
-
-	function test_jetpack_constants_can_override_previously_defined_constant() {
-		$test_version = '1.0.0';
-		Jetpack_Constants::set_constant( 'JETPACK__VERSION', $test_version );
-		$this->assertEquals( Jetpack_Constants::get_constant( 'JETPACK__VERSION' ), $test_version );
-	}
-
-	function test_jetpack_constants_override_to_null_gets_null() {
-		Jetpack_Constants::set_constant( 'JETPACK__VERSION', null );
-		$this->assertNull( Jetpack_Constants::get_constant( 'JETPACK__VERSION' ) );
-	}
-
-	// Jetpack_Constants::set_constant()
-
-	function test_jetpack_constants_set_constants_adds_to_set_constants_array() {
-		$key = 'TEST';
-		Jetpack_Constants::set_constant( $key, '1' );
-		$this->assertArrayHasKey( $key, Jetpack_Constants::$set_constants );
-		$this->assertEquals( '1', Jetpack_Constants::$set_constants[ $key ] );
-	}
-
-	// Jetpack_Constants::clear_constants()
-
-	function test_jetpack_constants_can_clear_all_constants() {
-		Jetpack_Constants::set_constant( 'JETPACK__VERSION', '1.0.0' );
-		Jetpack_Constants::clear_constants();
-		$this->assertEmpty( Jetpack_Constants::$set_constants );
-	}
-
-	// Jetpack_Constants::clear_single_constant()
-
-	function test_jetpack_constants_can_clear_single_constant() {
-		Jetpack_Constants::set_constant( 'FIRST', '1' );
-		Jetpack_Constants::set_constant( 'SECOND', '2' );
-
-		$this->assertCount( 2, Jetpack_Constants::$set_constants );
-
-		Jetpack_Constants::clear_single_constant( 'FIRST' );
-
-		$this->assertCount( 1, Jetpack_Constants::$set_constants );
-		$this->assertContains( 'SECOND', array_keys( Jetpack_Constants::$set_constants ) );
-	}
-
-	function test_jetpack_constants_can_clear_single_constant_when_null() {
-		Jetpack_Constants::set_constant( 'TEST', null );
-		$this->assertCount( 1, Jetpack_Constants::$set_constants );
-
-		Jetpack_Constants::clear_single_constant( 'TEST' );
-
-		$this->assertEmpty( Jetpack_Constants::$set_constants );
-	}
-
-	// Jetpack_Constant::is_true
-	function test_jetpack_constants_is_true_method() {
-		$this->assertFalse( Jetpack_Constants::is_true( 'FOO' ), 'unset constant returns true' );
-		Jetpack_Constants::set_constant( 'FOO', false );
-
-		$this->assertFalse( Jetpack_Constants::is_true( 'FOO' ), 'false constant returns true' );
-		Jetpack_Constants::set_constant( 'FOO', true );
-
-		$this->assertTrue( Jetpack_Constants::is_true( 'FOO' ), 'true constant returns false');
-	}
-}
diff --git a/tests/php/general/test-class.jetpack-options.php b/tests/php/general/test-class.jetpack-options.php
index 0241c7091574..83e360a44f60 100644
--- a/tests/php/general/test-class.jetpack-options.php
+++ b/tests/php/general/test-class.jetpack-options.php
@@ -1,5 +1,7 @@
 <?php
 
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
 class WP_Test_Jetpack_Options extends WP_UnitTestCase {
 
 	function tearDown() {
@@ -48,7 +50,7 @@ function test_raw_option_update_will_bypass_wp_cache_and_filters() {
 
 
 	function test_raw_option_with_constant_does_not_by_pass_wp_cache_filters() {
-		Jetpack_Constants::set_constant( 'JETPACK_DISABLE_RAW_OPTIONS', true);
+		Constants_Manager::set_constant( 'JETPACK_DISABLE_RAW_OPTIONS', true);
 		$option_name = 'test_option_with_constant';
 		add_action( 'added_option', array( $this, 'cache_option' ), 10, 2 );
 		add_filter( 'option_' . $option_name, array( $this, 'get_test_option_from_cache' ) );
@@ -60,7 +62,7 @@ function test_raw_option_with_constant_does_not_by_pass_wp_cache_filters() {
 
 		remove_action( 'added_option', array( $this, 'cache_option' ), 10, 2 );
 		remove_filter( 'option_'. $option_name, array( $this, 'get_test_option_from_cache' ) );
-		Jetpack_Constants::clear_single_constant( 'JETPACK_DISABLE_RAW_OPTIONS' );
+		Constants_Manager::clear_single_constant( 'JETPACK_DISABLE_RAW_OPTIONS' );
 	}
 
 	function test_raw_option_with_filter_does_not_by_pass_wp_cache_filters() {
diff --git a/tests/php/general/test_class.jetpack-data.php b/tests/php/general/test_class.jetpack-data.php
index 691ba32a4b49..d4f6db31e398 100644
--- a/tests/php/general/test_class.jetpack-data.php
+++ b/tests/php/general/test_class.jetpack-data.php
@@ -1,5 +1,7 @@
 <?php
 
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
 /**
  * @covers Jetpack_Data
  */
@@ -28,7 +30,7 @@ public function tearDown() {
 		Jetpack_Options::delete_option( 'user_tokens' );
 		Jetpack_Options::delete_option( 'master_user' );
 
-		Jetpack_Constants::clear_constants();
+		Constants_Manager::clear_constants();
 
 		parent::tearDown();
 	}
@@ -48,7 +50,7 @@ public function test_get_access_token_with_no_args_returns_blog_token() {
 	}
 
 	public function test_get_access_token_with_no_args_returns_defined_blog_token_when_constant_set() {
-		Jetpack_Constants::set_constant( 'JETPACK_BLOG_TOKEN', self::DEFINED );
+		Constants_Manager::set_constant( 'JETPACK_BLOG_TOKEN', self::DEFINED );
 
 		$token = Jetpack_Data::get_access_token();
 
@@ -58,7 +60,7 @@ public function test_get_access_token_with_no_args_returns_defined_blog_token_wh
 
 	public function test_get_access_token_with_no_args_returns_defined_blog_token_when_constant_set_and_no_stored_token() {
 		Jetpack_Options::delete_option( 'blog_token' );
-		Jetpack_Constants::set_constant( 'JETPACK_BLOG_TOKEN', self::DEFINED );
+		Constants_Manager::set_constant( 'JETPACK_BLOG_TOKEN', self::DEFINED );
 
 		$token = Jetpack_Data::get_access_token();
 
@@ -68,7 +70,7 @@ public function test_get_access_token_with_no_args_returns_defined_blog_token_wh
 
 
 	public function test_get_access_token_with_stored_key_returns_stored_blog_token() {
-		Jetpack_Constants::set_constant( 'JETPACK_BLOG_TOKEN', self::DEFINED );
+		Constants_Manager::set_constant( 'JETPACK_BLOG_TOKEN', self::DEFINED );
 
 		$token = Jetpack_Data::get_access_token( false, '12345' );
 
@@ -77,7 +79,7 @@ public function test_get_access_token_with_stored_key_returns_stored_blog_token(
 	}
 
 	public function test_get_access_token_with_magic_key_returns_stored_blog_token() {
-		Jetpack_Constants::set_constant( 'JETPACK_BLOG_TOKEN', self::DEFINED );
+		Constants_Manager::set_constant( 'JETPACK_BLOG_TOKEN', self::DEFINED );
 
 		$token = Jetpack_Data::get_access_token( false, Jetpack_Data::MAGIC_NORMAL_TOKEN_KEY );
 
@@ -88,7 +90,7 @@ public function test_get_access_token_with_magic_key_returns_stored_blog_token()
 
 	public function test_get_access_token_with_magic_key_returns_defined_blog_token_if_it_looks_like_a_stored_token_and_no_stored_token() {
 		Jetpack_Options::delete_option( 'blog_token' );
-		Jetpack_Constants::set_constant( 'JETPACK_BLOG_TOKEN', self::STORED );
+		Constants_Manager::set_constant( 'JETPACK_BLOG_TOKEN', self::STORED );
 
 		$token = Jetpack_Data::get_access_token( false, Jetpack_Data::MAGIC_NORMAL_TOKEN_KEY );
 
@@ -97,7 +99,7 @@ public function test_get_access_token_with_magic_key_returns_defined_blog_token_
 	}
 
 	public function test_get_access_token_with_no_args_returns_first_defined_blog_token_when_constant_multi_set() {
-		Jetpack_Constants::set_constant( 'JETPACK_BLOG_TOKEN', self::DEFINED_MULTI );
+		Constants_Manager::set_constant( 'JETPACK_BLOG_TOKEN', self::DEFINED_MULTI );
 
 		$token = Jetpack_Data::get_access_token();
 
@@ -107,7 +109,7 @@ public function test_get_access_token_with_no_args_returns_first_defined_blog_to
 
 	public function test_get_access_token_with_no_args_returns_first_defined_blog_token_when_constant_multi_set_and_no_stored_token() {
 		Jetpack_Options::delete_option( 'blog_token' );
-		Jetpack_Constants::set_constant( 'JETPACK_BLOG_TOKEN', self::DEFINED_MULTI );
+		Constants_Manager::set_constant( 'JETPACK_BLOG_TOKEN', self::DEFINED_MULTI );
 
 		$token = Jetpack_Data::get_access_token();
 
@@ -116,7 +118,7 @@ public function test_get_access_token_with_no_args_returns_first_defined_blog_to
 	}
 
 	public function test_get_access_token_with_token_key_returns_matching_token_when_constant_multi_set() {
-		Jetpack_Constants::set_constant( 'JETPACK_BLOG_TOKEN', self::DEFINED_MULTI );
+		Constants_Manager::set_constant( 'JETPACK_BLOG_TOKEN', self::DEFINED_MULTI );
 
 		$token = Jetpack_Data::get_access_token( false, ';foo;' );
 
@@ -126,7 +128,7 @@ public function test_get_access_token_with_token_key_returns_matching_token_when
 
 	public function test_get_access_token_with_token_key_returns_matching_token_when_constant_multi_set_and_no_stored_token() {
 		Jetpack_Options::delete_option( 'blog_token' );
-		Jetpack_Constants::set_constant( 'JETPACK_BLOG_TOKEN', self::DEFINED_MULTI );
+		Constants_Manager::set_constant( 'JETPACK_BLOG_TOKEN', self::DEFINED_MULTI );
 
 		$token = Jetpack_Data::get_access_token( false, ';foo;' );
 
@@ -135,7 +137,7 @@ public function test_get_access_token_with_token_key_returns_matching_token_when
 	}
 
 	public function test_get_access_token_with_magic_key_returns_stored_token_when_constant_multi_set() {
-		Jetpack_Constants::set_constant( 'JETPACK_BLOG_TOKEN', self::DEFINED_MULTI );
+		Constants_Manager::set_constant( 'JETPACK_BLOG_TOKEN', self::DEFINED_MULTI );
 
 		$token = Jetpack_Data::get_access_token( false, Jetpack_Data::MAGIC_NORMAL_TOKEN_KEY );
 
@@ -145,7 +147,7 @@ public function test_get_access_token_with_magic_key_returns_stored_token_when_c
 
 	public function test_get_access_token_with_magic_key_returns_matching_token_when_constant_multi_set_and_no_stored_token() {
 		Jetpack_Options::delete_option( 'blog_token' );
-		Jetpack_Constants::set_constant( 'JETPACK_BLOG_TOKEN', self::DEFINED_MULTI );
+		Constants_Manager::set_constant( 'JETPACK_BLOG_TOKEN', self::DEFINED_MULTI );
 
 		$token = Jetpack_Data::get_access_token( false, Jetpack_Data::MAGIC_NORMAL_TOKEN_KEY );
 
@@ -154,7 +156,7 @@ public function test_get_access_token_with_magic_key_returns_matching_token_when
 	}
 
 	public function test_get_access_token_with_token_key_requires_full_key() {
-		Jetpack_Constants::set_constant( 'JETPACK_BLOG_TOKEN', self::DEFINED_MULTI );
+		Constants_Manager::set_constant( 'JETPACK_BLOG_TOKEN', self::DEFINED_MULTI );
 
 		$token = Jetpack_Data::get_access_token( false, ';fo' );
 
diff --git a/tests/php/general/test_class.jetpack.php b/tests/php/general/test_class.jetpack.php
index d70654c95b89..64917342b960 100644
--- a/tests/php/general/test_class.jetpack.php
+++ b/tests/php/general/test_class.jetpack.php
@@ -1,5 +1,7 @@
 <?php
 
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
 // Extend with a public constructor so that can be mocked in tests
 class MockJetpack extends Jetpack {
 	public function __construct() {
@@ -13,7 +15,7 @@ class WP_Test_Jetpack extends WP_UnitTestCase {
 
 	public function tearDown() {
 		parent::tearDown();
-		Jetpack_Constants::clear_constants();
+		Constants_Manager::clear_constants();
 	}
 
 	/**
@@ -379,17 +381,17 @@ function test_idc_optin_casts_to_bool() {
 	}
 
 	function test_idc_optin_true_when_constant_true() {
-		Jetpack_Constants::set_constant( 'JETPACK_SYNC_IDC_OPTIN', true );
+		Constants_Manager::set_constant( 'JETPACK_SYNC_IDC_OPTIN', true );
 		$this->assertTrue( Jetpack::sync_idc_optin() );
 	}
 
 	function test_idc_optin_false_when_constant_false() {
-		Jetpack_Constants::set_constant( 'JETPACK_SYNC_IDC_OPTIN', false );
+		Constants_Manager::set_constant( 'JETPACK_SYNC_IDC_OPTIN', false );
 		$this->assertFalse( Jetpack::sync_idc_optin() );
 	}
 
 	function test_idc_optin_filter_overrides_constant() {
-		Jetpack_Constants::set_constant( 'JETPACK_SYNC_IDC_OPTIN', true );
+		Constants_Manager::set_constant( 'JETPACK_SYNC_IDC_OPTIN', true );
 		add_filter( 'jetpack_sync_idc_optin', '__return_false' );
 		$this->assertFalse( Jetpack::sync_idc_optin() );
 		remove_filter( 'jetpack_sync_idc_optin', '__return_false' );
@@ -429,7 +431,7 @@ function test_sync_error_idc_validation_cleans_up_when_part_of_validation_fails(
 
 	function test_sync_error_idc_validation_returns_false_and_cleans_up_when_opted_out() {
 		Jetpack_Options::update_option( 'sync_error_idc', Jetpack::get_sync_error_idc_option() );
-		Jetpack_Constants::set_constant( 'JETPACK_SYNC_IDC_OPTIN', false );
+		Constants_Manager::set_constant( 'JETPACK_SYNC_IDC_OPTIN', false );
 
 		$this->assertFalse( Jetpack::validate_sync_error_idc_option() );
 		$this->assertFalse( Jetpack_Options::get_option( 'sync_error_idc' ) );
@@ -507,27 +509,27 @@ function test_is_staging_site_true_when_sync_error_idc_is_valid() {
 	}
 
 	function test_is_dev_version_true_with_alpha() {
-		Jetpack_Constants::set_constant( 'JETPACK__VERSION', '4.3.1-alpha' );
+		Constants_Manager::set_constant( 'JETPACK__VERSION', '4.3.1-alpha' );
 		$this->assertTrue( Jetpack::is_development_version() );
 	}
 
 	function test_is_dev_version_true_with_beta() {
-		Jetpack_Constants::set_constant( 'JETPACK__VERSION', '4.3-beta2' );
+		Constants_Manager::set_constant( 'JETPACK__VERSION', '4.3-beta2' );
 		$this->assertTrue( Jetpack::is_development_version() );
 	}
 
 	function test_is_dev_version_true_with_rc() {
-		Jetpack_Constants::set_constant( 'JETPACK__VERSION', '4.3-rc2' );
+		Constants_Manager::set_constant( 'JETPACK__VERSION', '4.3-rc2' );
 		$this->assertTrue( Jetpack::is_development_version() );
 	}
 
 	function test_is_dev_version_false_with_number_dot_number() {
-		Jetpack_Constants::set_constant( 'JETPACK__VERSION', '4.3' );
+		Constants_Manager::set_constant( 'JETPACK__VERSION', '4.3' );
 		$this->assertFalse( Jetpack::is_development_version() );
 	}
 
 	function test_is_dev_version_false_with_number_dot_number_dot_number() {
-		Jetpack_Constants::set_constant( 'JETPACK__VERSION', '4.3.1' );
+		Constants_Manager::set_constant( 'JETPACK__VERSION', '4.3.1' );
 		$this->assertFalse( Jetpack::is_development_version() );
 	}
 
@@ -798,7 +800,7 @@ function test_get_assumed_site_creation_date_only_admins() {
 	 * @dataProvider get_file_url_for_environment_data_provider
 	 */
 	function test_get_file_url_for_environment( $min_path, $non_min_path, $is_script_debug, $expected, $not_expected ) {
-		Jetpack_Constants::set_constant( 'SCRIPT_DEBUG', $is_script_debug );
+		Constants_Manager::set_constant( 'SCRIPT_DEBUG', $is_script_debug );
 		$file_url = Jetpack::get_file_url_for_environment( $min_path, $non_min_path );
 
 		$this->assertContains( $$expected, $file_url );
diff --git a/tests/php/modules/search/test-class.jetpack-search-helpers.php b/tests/php/modules/search/test-class.jetpack-search-helpers.php
index a4d91f0d425f..cc928fd9dff4 100644
--- a/tests/php/modules/search/test-class.jetpack-search-helpers.php
+++ b/tests/php/modules/search/test-class.jetpack-search-helpers.php
@@ -1,5 +1,7 @@
 <?php
 
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
 require_jetpack_file( 'modules/search/class.jetpack-search.php' );
 require_jetpack_file( 'modules/search/class.jetpack-search-helpers.php' );
 
@@ -49,7 +51,7 @@ function tearDown() {
 
 		unset( $GLOBALS['wp_customize'] );
 
-		Jetpack_Constants::clear_constants();
+		Constants_Manager::clear_constants();
 		remove_all_filters( 'jetpack_search_has_vip_index' );
 	}
 
@@ -407,7 +409,7 @@ function test_ensure_post_types_on_remove_url( $expected, $filters, $post_types
 	 */
 	public function test_site_has_vip_index( $expected, $constant = null, $filter = false ) {
 		if ( ! is_null( $constant ) ) {
-			Jetpack_Constants::set_constant( 'JETPACK_SEARCH_VIP_INDEX', $constant );
+			Constants_Manager::set_constant( 'JETPACK_SEARCH_VIP_INDEX', $constant );
 		}
 
 		if ( $filter ) {
@@ -421,7 +423,7 @@ public function test_site_has_vip_index( $expected, $constant = null, $filter =
 	 * @dataProvider get_max_posts_per_page_data
 	 */
 	public function test_get_max_posts_per_page( $expected, $has_vip_index ) {
-		Jetpack_Constants::set_constant( 'JETPACK_SEARCH_VIP_INDEX', $has_vip_index );
+		Constants_Manager::set_constant( 'JETPACK_SEARCH_VIP_INDEX', $has_vip_index );
 		$this->assertSame( $expected, Jetpack_Search_Helpers::get_max_posts_per_page() );
 	}
 
@@ -429,7 +431,7 @@ public function test_get_max_posts_per_page( $expected, $has_vip_index ) {
 	 * @dataProvider get_max_offset_data
 	 */
 	public function test_get_max_offset( $expected, $has_vip_index ) {
-		Jetpack_Constants::set_constant( 'JETPACK_SEARCH_VIP_INDEX', $has_vip_index );
+		Constants_Manager::set_constant( 'JETPACK_SEARCH_VIP_INDEX', $has_vip_index );
 		$this->assertSame( $expected, Jetpack_Search_Helpers::get_max_offset() );
 	}
 
diff --git a/tests/php/sync/test_class.jetpack-sync-callables.php b/tests/php/sync/test_class.jetpack-sync-callables.php
index 9d4b6aaaeffb..18ac8d4962e7 100644
--- a/tests/php/sync/test_class.jetpack-sync-callables.php
+++ b/tests/php/sync/test_class.jetpack-sync-callables.php
@@ -1,5 +1,7 @@
 <?php
 
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
 require_once dirname( __FILE__ ) . '/../../../sync/class.jetpack-sync-functions.php';
 
 require_once 'test_class.jetpack-sync-base.php';
@@ -654,8 +656,8 @@ function test_get_raw_url_by_option_bypasses_filters() {
 	}
 
 	function test_get_raw_url_by_constant_bypasses_filters() {
-		Jetpack_Constants::set_constant( 'WP_HOME', 'http://constanturl.com' );
-		Jetpack_Constants::set_constant( 'WP_SITEURL', 'http://constanturl.com' );
+		Constants_Manager::set_constant( 'WP_HOME', 'http://constanturl.com' );
+		Constants_Manager::set_constant( 'WP_SITEURL', 'http://constanturl.com' );
 		add_filter( 'option_home', array( $this, '__return_filtered_url' ) );
 		add_filter( 'option_siteurl', array( $this, '__return_filtered_url' ) );
 
@@ -669,7 +671,7 @@ function test_get_raw_url_by_constant_bypasses_filters() {
 
 		remove_filter( 'option_home', array( $this, '__return_filtered_url' ) );
 		remove_filter( 'option_siteurl', array( $this, '__return_filtered_url' ) );
-		Jetpack_Constants::clear_constants();
+		Constants_Manager::clear_constants();
 	}
 
 	function test_get_raw_url_returns_with_http_if_is_ssl() {
@@ -688,7 +690,7 @@ function test_get_raw_url_returns_with_http_if_is_ssl() {
 	}
 
 	function test_raw_home_url_is_https_when_is_ssl() {
-		Jetpack_Constants::set_constant( 'JETPACK_SYNC_USE_RAW_URL', true );
+		Constants_Manager::set_constant( 'JETPACK_SYNC_USE_RAW_URL', true );
 
 		$home_option = get_option( 'home' );
 
@@ -715,9 +717,9 @@ function test_user_can_stop_raw_urls() {
 		$this->assertTrue( 'http://filteredurl.com' !== Jetpack_Sync_Functions::home_url() );
 
 		// Now, without, which should return the filtered URL
-		Jetpack_Constants::set_constant( 'JETPACK_SYNC_USE_RAW_URL', false );
+		Constants_Manager::set_constant( 'JETPACK_SYNC_USE_RAW_URL', false );
 		$this->assertEquals( $this->__return_filtered_url(), Jetpack_Sync_Functions::home_url() );
-		Jetpack_Constants::clear_constants();
+		Constants_Manager::clear_constants();
 
 		remove_filter( 'option_home', array( $this, '__return_filtered_url' ) );
 		remove_filter( 'option_siteurl', array( $this, '__return_filtered_url' ) );
@@ -940,7 +942,7 @@ function mock_authenticated_xml_rpc() {
 		$_GET['signature'] = base64_encode( hash_hmac( 'sha1', $normalize , 'secret', true ) );
 
 		// call one of the authenticated endpoints
-		Jetpack_Constants::set_constant( 'XMLRPC_REQUEST', true );
+		Constants_Manager::set_constant( 'XMLRPC_REQUEST', true );
 		$jetpack = Jetpack::init();
 		$jetpack->xmlrpc_methods( array() );
 		$jetpack->require_jetpack_authentication();
@@ -948,7 +950,7 @@ function mock_authenticated_xml_rpc() {
 	}
 
 	function mock_authenticated_xml_rpc_cleanup( $user_id ) {
-		Jetpack_Constants::clear_constants();
+		Constants_Manager::clear_constants();
 		remove_filter( 'pre_option_jetpack_private_options', array( $this, 'mock_jetpack_private_options' ), 10 );
 
 		unset( $_GET['token'] );
diff --git a/tests/php/sync/test_class.jetpack-sync-integration.php b/tests/php/sync/test_class.jetpack-sync-integration.php
index 935f99714f89..5a44b6c563e2 100644
--- a/tests/php/sync/test_class.jetpack-sync-integration.php
+++ b/tests/php/sync/test_class.jetpack-sync-integration.php
@@ -1,5 +1,7 @@
 <?php
 
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
 class WP_Test_Jetpack_Sync_Integration extends WP_Test_Jetpack_Sync_Base {
 	function test_sending_empties_queue() {
 		$this->factory->post->create();
@@ -103,7 +105,7 @@ function test_loads_sender_if_listener_queues_actions() {
 	}
 
 	function test_do_not_load_sender_if_is_cron_and_cron_sync_disabled() {
-		Jetpack_Constants::set_constant( 'DOING_CRON', true );
+		Constants_Manager::set_constant( 'DOING_CRON', true );
 		$settings = Jetpack_Sync_Settings::get_settings();
 		$settings['sync_via_cron'] = 0;
 		Jetpack_Sync_Settings::update_settings( $settings );
@@ -113,7 +115,7 @@ function test_do_not_load_sender_if_is_cron_and_cron_sync_disabled() {
 
 		$this->assertNull( Jetpack_Sync_Actions::$sender );
 
-		Jetpack_Constants::clear_constants();
+		Constants_Manager::clear_constants();
 		Jetpack_Sync_Settings::reset_data();
 	}
 
diff --git a/tests/php/sync/test_class.jetpack-sync-module-protect.php b/tests/php/sync/test_class.jetpack-sync-module-protect.php
index 0b697bba0c5e..341bc6a1762d 100644
--- a/tests/php/sync/test_class.jetpack-sync-module-protect.php
+++ b/tests/php/sync/test_class.jetpack-sync-module-protect.php
@@ -1,5 +1,7 @@
 <?php
 
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
 /**
  * Test pluggable functionality for bruteprotect
  */
@@ -26,9 +28,9 @@ function test_do_not_send_failed_login_message() {
 		$user_id = $this->factory->user->create();
 
 		$user = get_userdata( $user_id );
-		Jetpack_Constants::set_constant( 'XMLRPC_REQUEST', true ); // fake xmlrpc request
+		Constants_Manager::set_constant( 'XMLRPC_REQUEST', true ); // fake xmlrpc request
 		Jetpack_Protect_Module::instance()->log_failed_attempt( $user->user_email );
-		Jetpack_Constants::clear_single_constant( 'XMLRPC_REQUEST' );
+		Constants_Manager::clear_single_constant( 'XMLRPC_REQUEST' );
 		$this->sender->do_sync();
 
 		$action = $this->server_event_storage->get_most_recent_event( 'jetpack_valid_failed_login_attempt' );
diff --git a/tests/php/sync/test_class.jetpack-sync-plugins-updates.php b/tests/php/sync/test_class.jetpack-sync-plugins-updates.php
index 5c9d62376a17..b13fafe05915 100644
--- a/tests/php/sync/test_class.jetpack-sync-plugins-updates.php
+++ b/tests/php/sync/test_class.jetpack-sync-plugins-updates.php
@@ -1,5 +1,7 @@
 <?php
 
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
 if ( ! class_exists( 'WP_Test_Jetpack_Sync_Plugins' ) ) {
 	$sync_dir        = dirname( __FILE__ );
 	require_once $sync_dir . '/test_class.jetpack-sync-plugins.php';
@@ -30,7 +32,7 @@ public function setUp() {
 
 	public function tearDown() {
 		parent::tearDown();
-		Jetpack_Constants::clear_constants();
+		Constants_Manager::clear_constants();
 	}
 
 	public static function setUpBeforeClass() {
@@ -159,7 +161,7 @@ function test_updating_error_with_autoupdate_constant_results_in_proper_state()
 			'api'    => '',
 		);
 
-		Jetpack_Constants::set_constant( 'JETPACK_PLUGIN_AUTOUPDATE', true );
+		Constants_Manager::set_constant( 'JETPACK_PLUGIN_AUTOUPDATE', true );
 
 		$this->set_error();
 		$this->update_bulk_plugins( new WP_Ajax_Upgrader_Skin( $plugin_defaults ) );
@@ -179,7 +181,7 @@ function test_updating_with_autoupdate_constant_results_in_proper_state() {
 			'api'    => '',
 		);
 
-		Jetpack_Constants::set_constant( 'JETPACK_PLUGIN_AUTOUPDATE', true );
+		Constants_Manager::set_constant( 'JETPACK_PLUGIN_AUTOUPDATE', true );
 		$this->update_bulk_plugins( new WP_Ajax_Upgrader_Skin( $plugin_defaults ) );
 		$this->sender->do_sync();
 		$updated_plugin = $this->server_event_storage->get_most_recent_event( 'jetpack_plugins_updated' );
diff --git a/tests/php/sync/test_class.jetpack-sync-posts.php b/tests/php/sync/test_class.jetpack-sync-posts.php
index 898cd2087ce4..6f1aa1c4b86c 100644
--- a/tests/php/sync/test_class.jetpack-sync-posts.php
+++ b/tests/php/sync/test_class.jetpack-sync-posts.php
@@ -1,5 +1,7 @@
 <?php
 
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
 /**
  * Testing CRUD on Posts
  */
@@ -45,7 +47,7 @@ public function test_add_post_syncs_request_is_auto_save() {
 		$event = $this->server_event_storage->get_most_recent_event( 'jetpack_sync_save_post' );
 		$this->assertFalse( $event->args[3]['is_auto_save'] );
 
-		Jetpack_Constants::set_constant( 'DOING_AUTOSAVE', true );
+		Constants_Manager::set_constant( 'DOING_AUTOSAVE', true );
 
 		//Performing sync here (even though setup() does it) to sync REQUEST_URI
 		$user_id = $this->factory->user->create();
diff --git a/tests/php/sync/test_class.jetpack-sync-updates.php b/tests/php/sync/test_class.jetpack-sync-updates.php
index d61fb0c3e0de..257d50c4455e 100644
--- a/tests/php/sync/test_class.jetpack-sync-updates.php
+++ b/tests/php/sync/test_class.jetpack-sync-updates.php
@@ -1,5 +1,7 @@
 <?php
 
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
+
 /**
  * Testing Updates Sync
  */
@@ -245,11 +247,11 @@ public function test_update_core_successfully_sync_action_using_wpcom_rest_api()
 		global $wp_version, $pagenow;
 
 		$this->assertFalse( $pagenow === 'update-core.php' );
-		Jetpack_Constants::set_constant( 'REST_API_REQUEST', true );
+		Constants_Manager::set_constant( 'REST_API_REQUEST', true );
 		Jetpack_Sync_Modules::get_module( "updates" )->update_core( 'new_version' );
 		$this->sender->do_sync();
 
-		Jetpack_Constants::clear_single_constant( 'REST_API_REQUEST' );
+		Constants_Manager::clear_single_constant( 'REST_API_REQUEST' );
 
 		$autoupdate_event = $this->server_event_storage->get_most_recent_event( 'jetpack_sync_core_autoupdated_successfully' );
 		$event = $this->server_event_storage->get_most_recent_event( 'jetpack_sync_core_updated_successfully' );
diff --git a/tests/php/sync/test_class.jetpack-sync-users.php b/tests/php/sync/test_class.jetpack-sync-users.php
index 8b8357e95885..d0d82007a4c8 100644
--- a/tests/php/sync/test_class.jetpack-sync-users.php
+++ b/tests/php/sync/test_class.jetpack-sync-users.php
@@ -1,5 +1,6 @@
 <?php
 
+use Automattic\Jetpack\Constants\Manager as Constants_Manager;
 
 /**
  * Testing CRUD on Users
@@ -710,12 +711,12 @@ public function test_no_save_user_sync_action_when_creating_user() {
 	public function test_invite_user_sync_invite_event() {
 		$this->server_event_storage->reset();
 		// Fake it till you make it
-		Jetpack_Constants::set_constant( 'JETPACK_INVITE_ACCEPTED', true );
+		Constants_Manager::set_constant( 'JETPACK_INVITE_ACCEPTED', true );
 		// We modify the input here to mimick the same call structure of the update user endpoint.
 		Jetpack_SSO_Helpers::generate_user( $this->get_invite_user_data() );
 		$this->sender->do_sync();
 
-		Jetpack_Constants::clear_constants();
+		Constants_Manager::clear_constants();
 
 		$event = $this->server_event_storage->get_most_recent_event( 'jetpack_sync_save_user' );
 		$this->assertFalse( $event );
@@ -729,12 +730,12 @@ public function test_invite_user_sync_invite_event() {
 	public function test_invite_user_sync_invite_event_false() {
 		$this->server_event_storage->reset();
 		// Fake it till we make it
-		Jetpack_Constants::set_constant( 'JETPACK_INVITE_ACCEPTED', false );
+		Constants_Manager::set_constant( 'JETPACK_INVITE_ACCEPTED', false );
 		// We modify the input here to mimick the same call structure of the update user endpoint.
 		Jetpack_SSO_Helpers::generate_user( $this->get_invite_user_data() );
 		$this->sender->do_sync();
 
-		Jetpack_Constants::clear_constants();
+		Constants_Manager::clear_constants();
 
 		$event = $this->server_event_storage->get_most_recent_event( 'jetpack_sync_save_user' );
 		$this->assertFalse( $event );