diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9ee4ea3aa1be..3b5434b0467c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -27,6 +27,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - _Upgrade (experimental)_: Ensure CSS before a layer stays unlayered when running codemods ([#14596](https://github.com/tailwindlabs/tailwindcss/pull/14596))
 - _Upgrade (experimental)_: Resolve issues where some prefixed candidates were not properly migrated ([#14600](https://github.com/tailwindlabs/tailwindcss/pull/14600))
 - _Upgrade (experimental)_: Migrate `@media screen(…)` when running codemods ([#14603](https://github.com/tailwindlabs/tailwindcss/pull/14603))
+- _Upgrade (experimental)_: Inject `@config "…"` when a `tailwind.config.{js,ts,…}` is detected ([#14635](https://github.com/tailwindlabs/tailwindcss/pull/14635))
 
 ## [4.0.0-alpha.26] - 2024-10-03
 
diff --git a/integrations/upgrade/index.test.ts b/integrations/upgrade/index.test.ts
index 8835c5483f36..b529d7a6299e 100644
--- a/integrations/upgrade/index.test.ts
+++ b/integrations/upgrade/index.test.ts
@@ -39,7 +39,9 @@ test(
       <div class="flex! sm:block! bg-linear-to-t bg-[var(--my-red)]"></div>
 
       --- ./src/input.css ---
-      @import 'tailwindcss';"
+      @import 'tailwindcss';
+      @config "../tailwind.config.js";
+      "
     `)
 
     let packageJsonContent = await fs.read('package.json')
@@ -95,9 +97,12 @@ test(
       --- ./src/input.css ---
       @import 'tailwindcss' prefix(tw);
 
+      @config "../tailwind.config.js";
+
       .btn {
         @apply tw:rounded-md! tw:px-2 tw:py-1 tw:bg-blue-500 tw:text-white;
-      }"
+      }
+      "
     `)
   },
 )
@@ -140,6 +145,8 @@ test(
       --- ./src/index.css ---
       @import 'tailwindcss';
 
+      @config "../tailwind.config.js";
+
       .a {
         @apply flex;
       }
@@ -150,7 +157,8 @@ test(
 
       .c {
         @apply flex! flex-col! items-center!;
-      }"
+      }
+      "
     `)
   },
 )
@@ -193,6 +201,8 @@ test(
       --- ./src/index.css ---
       @import 'tailwindcss';
 
+      @config "../tailwind.config.js";
+
       @layer base {
         html {
           color: #333;
@@ -203,7 +213,8 @@ test(
         .btn {
           color: red;
         }
-      }"
+      }
+      "
     `)
   },
 )
@@ -251,6 +262,8 @@ test(
       --- ./src/index.css ---
       @import 'tailwindcss';
 
+      @config "../tailwind.config.js";
+
       @utility btn {
         @apply rounded-md px-2 py-1 bg-blue-500 text-white;
       }
@@ -261,7 +274,8 @@ test(
         }
         -ms-overflow-style: none;
         scrollbar-width: none;
-      }"
+      }
+      "
     `)
   },
 )
@@ -533,7 +547,8 @@ test(
       <div class="flex"></div>
 
       --- ./src/other.html ---
-      <div class="tw:flex"></div>"
+      <div class="tw:flex"></div>
+      "
     `)
   },
 )
@@ -573,7 +588,8 @@ test(
       <div class="tw:bg-linear-to-t"></div>
 
       --- ./src/other.html ---
-      <div class="bg-gradient-to-t"></div>"
+      <div class="bg-gradient-to-t"></div>
+      "
     `)
   },
 )
@@ -615,6 +631,7 @@ test(
       --- ./src/index.css ---
       @import 'tailwindcss';
       @import './utilities.css';
+      @config "../tailwind.config.js";
 
       --- ./src/utilities.css ---
       @utility no-scrollbar {
@@ -623,7 +640,8 @@ test(
         }
         -ms-overflow-style: none;
         scrollbar-width: none;
-      }"
+      }
+      "
     `)
   },
 )
@@ -730,6 +748,7 @@ test(
       @import './c.1.css' layer(utilities);
       @import './c.1.utilities.css';
       @import './d.1.css';
+      @config "../tailwind.config.js";
 
       --- ./src/a.1.css ---
       @import './a.1.utilities.css'
@@ -804,7 +823,8 @@ test(
       --- ./src/d.4.css ---
       @utility from-a-4 {
         color: blue;
-      }"
+      }
+      "
     `)
   },
 )
@@ -862,14 +882,141 @@ test(
       --- ./src/root.1.css ---
       @import 'tailwindcss/utilities' layer(utilities);
       @import './a.1.css' layer(utilities);
+      @config "../tailwind.config.js";
 
       --- ./src/root.2.css ---
       @import 'tailwindcss/utilities' layer(utilities);
       @import './a.1.css' layer(components);
+      @config "../tailwind.config.js";
+
+      --- ./src/root.3.css ---
+      @import 'tailwindcss/utilities' layer(utilities);
+      @import './a.1.css' layer(utilities);
+      @config "../tailwind.config.js";
+      "
+    `)
+  },
+)
+
+test(
+  'injecting `@config` when a tailwind.config.{js,ts,…} is detected',
+  {
+    fs: {
+      'package.json': json`
+        {
+          "dependencies": {
+            "@tailwindcss/upgrade": "workspace:^"
+          }
+        }
+      `,
+      'tailwind.config.ts': js`
+        export default {
+          content: ['./src/**/*.{html,js}'],
+        }
+      `,
+      'src/index.html': html`
+        <h1>🤠👋</h1>
+        <div class="!flex sm:!block bg-gradient-to-t bg-[--my-red]"></div>
+      `,
+      'src/root.1.css': css`
+        /* Inject missing @config */
+        @tailwind base;
+        @tailwind components;
+        @tailwind utilities;
+      `,
+      'src/root.2.css': css`
+        /* Already contains @config */
+        @tailwind base;
+        @tailwind components;
+        @tailwind utilities;
+        @config "../tailwind.config.js";
+      `,
+      'src/root.3.css': css`
+        /* Inject missing @config above first @theme */
+        @tailwind base;
+        @tailwind components;
+        @tailwind utilities;
+
+        @variant hocus (&:hover, &:focus);
+
+        @theme {
+          --color-red-500: #f00;
+        }
+
+        @theme {
+          --color-blue-500: #00f;
+        }
+      `,
+      'src/root.4.css': css`
+        /* Inject missing @config due to nested imports with tailwind imports */
+        @import './root.4/base.css';
+        @import './root.4/utilities.css';
+      `,
+      'src/root.4/base.css': css`@import 'tailwindcss/base';`,
+      'src/root.4/utilities.css': css`@import 'tailwindcss/utilities';`,
+
+      'src/root.5.css': css`@import './root.5/tailwind.css';`,
+      'src/root.5/tailwind.css': css`
+        /* Inject missing @config in this file, due to full import */
+        @import 'tailwindcss';
+      `,
+    },
+  },
+  async ({ exec, fs }) => {
+    await exec('npx @tailwindcss/upgrade --force')
+
+    expect(await fs.dumpFiles('./src/**/*.{html,css}')).toMatchInlineSnapshot(`
+      "
+      --- ./src/index.html ---
+      <h1>🤠👋</h1>
+      <div class="flex! sm:block! bg-linear-to-t bg-[var(--my-red)]"></div>
+
+      --- ./src/root.1.css ---
+      /* Inject missing @config */
+      @import 'tailwindcss';
+      @config "../tailwind.config.ts";
+
+      --- ./src/root.2.css ---
+      /* Already contains @config */
+      @import 'tailwindcss';
+      @config "../tailwind.config.js";
 
       --- ./src/root.3.css ---
+      /* Inject missing @config above first @theme */
+      @import 'tailwindcss';
+      @config "../tailwind.config.ts";
+
+      @variant hocus (&:hover, &:focus);
+
+      @theme {
+        --color-red-500: #f00;
+      }
+
+      @theme {
+        --color-blue-500: #00f;
+      }
+
+      --- ./src/root.4.css ---
+      /* Inject missing @config due to nested imports with tailwind imports */
+      @import './root.4/base.css';
+      @import './root.4/utilities.css';
+      @config "../tailwind.config.ts";
+
+      --- ./src/root.5.css ---
+      @import './root.5/tailwind.css';
+
+      --- ./src/root.4/base.css ---
+      @import 'tailwindcss/theme' layer(theme);
+      @import 'tailwindcss/preflight' layer(base);
+
+      --- ./src/root.4/utilities.css ---
       @import 'tailwindcss/utilities' layer(utilities);
-      @import './a.1.css' layer(utilities);"
+
+      --- ./src/root.5/tailwind.css ---
+      /* Inject missing @config in this file, due to full import */
+      @import 'tailwindcss';
+      @config "../../tailwind.config.ts";
+      "
     `)
   },
 )
diff --git a/integrations/utils.ts b/integrations/utils.ts
index ce700bce7f7a..2afc35ed038a 100644
--- a/integrations/utils.ts
+++ b/integrations/utils.ts
@@ -330,7 +330,8 @@ export function test(
                 return a[0].localeCompare(z[0])
               })
               .map(([file, content]) => `--- ${file} ---\n${content || '<EMPTY>'}`)
-              .join('\n\n')}`
+              .join('\n\n')
+              .trim()}\n`
           },
           async expectFileToContain(filePath, contents) {
             return retryAssertion(async () => {
diff --git a/packages/@tailwindcss-upgrade/src/codemods/migrate-at-config.ts b/packages/@tailwindcss-upgrade/src/codemods/migrate-at-config.ts
new file mode 100644
index 000000000000..4e1f10663b27
--- /dev/null
+++ b/packages/@tailwindcss-upgrade/src/codemods/migrate-at-config.ts
@@ -0,0 +1,101 @@
+import path from 'node:path'
+import { AtRule, type Plugin, type Root } from 'postcss'
+import { normalizePath } from '../../../@tailwindcss-node/src/normalize-path'
+import type { Stylesheet } from '../stylesheet'
+import { walk, WalkAction } from '../utils/walk'
+
+export function migrateAtConfig(
+  sheet: Stylesheet,
+  { configFilePath }: { configFilePath: string },
+): Plugin {
+  function injectInto(sheet: Stylesheet) {
+    let root = sheet.root
+
+    // We don't have a sheet with a file path
+    if (!sheet.file) return
+
+    // Skip if there is already a `@config` directive
+    {
+      let hasConfig = false
+      root.walkAtRules('config', () => {
+        hasConfig = true
+        return false
+      })
+      if (hasConfig) return
+    }
+
+    // Figure out the path to the config file
+    let sheetPath = sheet.file
+    let configPath = configFilePath
+
+    let relative = path.relative(path.dirname(sheetPath), configPath)
+    if (relative[0] !== '.') {
+      relative = `./${relative}`
+    }
+    // Ensure relative is a posix style path since we will merge it with the
+    // glob.
+    relative = normalizePath(relative)
+
+    // Inject the `@config` in a sensible place
+    // 1. Below the last `@import`
+    // 2. At the top of the file
+    let locationNode = null as AtRule | null
+
+    walk(root, (node) => {
+      if (node.type === 'atrule' && node.name === 'import') {
+        locationNode = node
+      }
+
+      return WalkAction.Skip
+    })
+
+    let configNode = new AtRule({ name: 'config', params: `"${relative}"` })
+
+    if (!locationNode) {
+      root.prepend(configNode)
+    } else if (locationNode.name === 'import') {
+      locationNode.after(configNode)
+    }
+  }
+
+  function migrate(root: Root) {
+    // We can only migrate if there is an `@import "tailwindcss"` (or sub-import)
+    let hasTailwindImport = false
+    let hasFullTailwindImport = false
+    root.walkAtRules('import', (node) => {
+      if (node.params.match(/['"]tailwindcss['"]/)) {
+        hasTailwindImport = true
+        hasFullTailwindImport = true
+        return false
+      } else if (node.params.match(/['"]tailwindcss\/.*?['"]/)) {
+        hasTailwindImport = true
+      }
+    })
+
+    if (!hasTailwindImport) return
+
+    // - If a full `@import "tailwindcss"` is present, we can inject the
+    //   `@config` directive directly into this stylesheet.
+    // - If we are the root file (no parents), then we can inject the `@config`
+    //   directive directly into this file as well.
+    if (hasFullTailwindImport || sheet.parents.size <= 0) {
+      injectInto(sheet)
+      return
+    }
+
+    // Otherwise, if we are not the root file, we need to inject the `@config`
+    // into the root file.
+    if (sheet.parents.size > 0) {
+      for (let parent of sheet.ancestors()) {
+        if (parent.parents.size === 0) {
+          injectInto(parent)
+        }
+      }
+    }
+  }
+
+  return {
+    postcssPlugin: '@tailwindcss/upgrade/migrate-at-config',
+    OnceExit: migrate,
+  }
+}
diff --git a/packages/@tailwindcss-upgrade/src/codemods/migrate-missing-layers.ts b/packages/@tailwindcss-upgrade/src/codemods/migrate-missing-layers.ts
index 6d3de9fe6c33..35050b932968 100644
--- a/packages/@tailwindcss-upgrade/src/codemods/migrate-missing-layers.ts
+++ b/packages/@tailwindcss-upgrade/src/codemods/migrate-missing-layers.ts
@@ -10,7 +10,13 @@ export function migrateMissingLayers(): Plugin {
     root.each((node) => {
       if (node.type === 'atrule') {
         // Known Tailwind directives that should not be inside a layer.
-        if (node.name === 'theme' || node.name === 'utility') {
+        if (
+          node.name === 'config' ||
+          node.name === 'source' ||
+          node.name === 'theme' ||
+          node.name === 'utility' ||
+          node.name === 'variant'
+        ) {
           if (bucket.length > 0) {
             buckets.push([lastLayer, bucket.splice(0)])
           }
diff --git a/packages/@tailwindcss-upgrade/src/index.test.ts b/packages/@tailwindcss-upgrade/src/index.test.ts
index fb5457774f88..d4763d375ee8 100644
--- a/packages/@tailwindcss-upgrade/src/index.test.ts
+++ b/packages/@tailwindcss-upgrade/src/index.test.ts
@@ -1,5 +1,6 @@
 import { __unstable__loadDesignSystem } from '@tailwindcss/node'
 import dedent from 'dedent'
+import path from 'node:path'
 import postcss from 'postcss'
 import { expect, it } from 'vitest'
 import { formatNodes } from './codemods/format-nodes'
@@ -13,7 +14,13 @@ let designSystem = await __unstable__loadDesignSystem(
   `,
   { base: __dirname },
 )
-let config = { designSystem, userConfig: {}, newPrefix: null }
+
+let config = {
+  designSystem,
+  userConfig: {},
+  newPrefix: null,
+  configFilePath: path.resolve(__dirname, './tailwind.config.js'),
+}
 
 function migrate(input: string, config: any) {
   return migrateContents(input, config, expect.getState().testPath)
@@ -87,6 +94,8 @@ it('should migrate a stylesheet', async () => {
   ).toMatchInlineSnapshot(`
     "@import 'tailwindcss';
 
+    @config "./tailwind.config.js";
+
     @layer base {
       html {
         overflow: hidden;
@@ -138,7 +147,8 @@ it('should migrate a stylesheet (with imports)', async () => {
     "@import 'tailwindcss';
     @import './my-base.css' layer(base);
     @import './my-components.css' layer(components);
-    @import './my-utilities.css' layer(utilities);"
+    @import './my-utilities.css' layer(utilities);
+    @config "./tailwind.config.js";"
   `)
 })
 
@@ -163,6 +173,7 @@ it('should migrate a stylesheet (with preceding rules that should be wrapped in
     @layer foo, bar, baz;
     /**! My license comment */
     @import 'tailwindcss';
+    @config "./tailwind.config.js";
     @layer base {
       html {
         color: red;
diff --git a/packages/@tailwindcss-upgrade/src/migrate.ts b/packages/@tailwindcss-upgrade/src/migrate.ts
index 0b32cace52c0..7ce0c1bb1f6a 100644
--- a/packages/@tailwindcss-upgrade/src/migrate.ts
+++ b/packages/@tailwindcss-upgrade/src/migrate.ts
@@ -5,6 +5,7 @@ import type { DesignSystem } from '../../tailwindcss/src/design-system'
 import { DefaultMap } from '../../tailwindcss/src/utils/default-map'
 import { segment } from '../../tailwindcss/src/utils/segment'
 import { migrateAtApply } from './codemods/migrate-at-apply'
+import { migrateAtConfig } from './codemods/migrate-at-config'
 import { migrateAtLayerUtilities } from './codemods/migrate-at-layer-utilities'
 import { migrateMediaScreen } from './codemods/migrate-media-screen'
 import { migrateMissingLayers } from './codemods/migrate-missing-layers'
@@ -17,6 +18,7 @@ export interface MigrateOptions {
   newPrefix: string | null
   designSystem: DesignSystem
   userConfig: Config
+  configFilePath: string
 }
 
 export async function migrateContents(
@@ -35,6 +37,7 @@ export async function migrateContents(
     .use(migrateAtLayerUtilities(stylesheet))
     .use(migrateMissingLayers())
     .use(migrateTailwindDirectives(options))
+    .use(migrateAtConfig(stylesheet, options))
     .process(stylesheet.root, { from: stylesheet.file ?? undefined })
 }
 
diff --git a/packages/@tailwindcss-upgrade/src/template/prepare-config.ts b/packages/@tailwindcss-upgrade/src/template/prepare-config.ts
index c9667f455352..b76f8990b2c5 100644
--- a/packages/@tailwindcss-upgrade/src/template/prepare-config.ts
+++ b/packages/@tailwindcss-upgrade/src/template/prepare-config.ts
@@ -22,6 +22,7 @@ export async function prepareConfig(
   designSystem: DesignSystem
   globs: { base: string; pattern: string }[]
   userConfig: Config
+  configFilePath: string
 
   newPrefix: string | null
 }> {
@@ -57,7 +58,13 @@ export async function prepareConfig(
       __unstable__loadDesignSystem(input, { base: __dirname }),
     ])
 
-    return { designSystem, globs: compiler.globs, userConfig, newPrefix }
+    return {
+      designSystem,
+      globs: compiler.globs,
+      userConfig,
+      newPrefix,
+      configFilePath: fullConfigPath,
+    }
   } catch (e: any) {
     error('Could not load the configuration file: ' + e.message)
     process.exit(1)