diff --git a/lib/class-wp-theme-json-gutenberg.php b/lib/class-wp-theme-json-gutenberg.php
index 7dd2c94bf30889..3eadb007f3d36c 100644
--- a/lib/class-wp-theme-json-gutenberg.php
+++ b/lib/class-wp-theme-json-gutenberg.php
@@ -724,7 +724,11 @@ private function get_block_classes( $style_nodes ) {
 			}
 
 			if ( self::ROOT_BLOCK_SELECTOR === $selector ) {
-				$block_rules .= 'body { margin: 0; }';
+				// If the root body node has margin set in its spacing property
+				// then avoid setting a default margin.
+				if ( ! isset( $node['spacing']['margin'] ) ) {
+					$block_rules .= 'body { margin: 0; }';
+				}
 				$block_rules .= '.wp-site-blocks > .alignleft { float: left; margin-right: 2em; }';
 				$block_rules .= '.wp-site-blocks > .alignright { float: right; margin-left: 2em; }';
 				$block_rules .= '.wp-site-blocks > .aligncenter { justify-content: center; margin-left: auto; margin-right: auto; }';