diff --git a/.github/workflows/integration-tests.yml b/.github/workflows/integration-tests.yml
index 457c38424..fd3c1a361 100644
--- a/.github/workflows/integration-tests.yml
+++ b/.github/workflows/integration-tests.yml
@@ -37,6 +37,7 @@ jobs:
           - postcss
           - oxide
           - webpack
+          - turbopack
 
         # Exclude windows and macos from being built on feature branches
         run-all:
diff --git a/CHANGELOG.md b/CHANGELOG.md
index c56103cd8..9044a9c33 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -54,6 +54,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Canonicalization: prevent `collapse: true` from taking exponentially longer when classes are linked through a utility that sets many properties (e.g. `sr-only`) ([#20558](https://github.com/tailwindlabs/tailwindcss/pull/20558))
 - Ensure `&` is substituted inside `:-moz-any(…)` and `:-webkit-any(…)` in custom variants (e.g. `@custom-variant firefox (:-moz-any(&));`) ([#20486](https://github.com/tailwindlabs/tailwindcss/pull/20486))
 - Ensure the standalone macOS binaries have a valid code signature, so macOS 27 no longer kills them at launch ([#20578](https://github.com/tailwindlabs/tailwindcss/pull/20578))
+- Ensure `@tailwindcss/turbopack` and `@tailwindcss/webpack` rebuild when the input CSS changes (e.g. `next dev` with Turbopack no longer serves the CSS of the previous edit) ([#20582](https://github.com/tailwindlabs/tailwindcss/pull/20582))
 
 ## [4.3.3] - 2026-07-16
 
diff --git a/integrations/turbopack/loader.test.ts b/integrations/turbopack/loader.test.ts
index 755cd60df..c5f4fec81 100644
--- a/integrations/turbopack/loader.test.ts
+++ b/integrations/turbopack/loader.test.ts
@@ -81,3 +81,99 @@ test(
     })
   },
 )
+
+// https://github.com/tailwindlabs/tailwindcss/issues/20581
+test(
+  '@tailwindcss/turbopack loader picks up changes to the CSS file (dev)',
+  {
+    timeout: 120_000,
+    fs: {
+      'package.json': json`
+        {
+          "dependencies": {
+            "next": "^16.2.7",
+            "react": "^19.2.7",
+            "react-dom": "^19.2.7",
+            "tailwindcss": "workspace:^",
+            "@tailwindcss/turbopack": "workspace:^"
+          }
+        }
+      `,
+      'next.config.mjs': js`
+        export default {
+          turbopack: {
+            rules: {
+              '*.css': {
+                loaders: ['@tailwindcss/turbopack'],
+                as: '*.css',
+              },
+            },
+          },
+        }
+      `,
+      'app/layout.js': jsx`
+        import './globals.css'
+
+        export default function RootLayout({ children }) {
+          return (
+            <html>
+              <body>{children}</body>
+            </html>
+          )
+        }
+      `,
+      'app/page.js': jsx`
+        export default function Page() {
+          return <div className="flex">Hello, Next.js!</div>
+        }
+      `,
+      'app/globals.css': css`
+        @import 'tailwindcss';
+
+        .custom-1 {
+          color: red;
+        }
+      `,
+    },
+  },
+  async ({ fs, spawn, expect }) => {
+    let process = await spawn('pnpm next dev')
+
+    let url = ''
+    await process.onStdout((message) => {
+      let match = /Local:\s*(http.*)/.exec(message)
+      if (match) url = match[1]
+      return Boolean(url)
+    })
+
+    await process.onStdout((message) => message.includes('Ready in'))
+
+    await retryAssertion(async () => {
+      let styles = await fetchStyles(url)
+      expect(styles).toContain(candidate`flex`)
+      expect(styles).toContain(candidate`custom-1`)
+    })
+
+    // Turbopack can run the loader with the previous version of the CSS file
+    // first, which must not leave the served CSS one edit behind
+    for (let i = 2; i <= 4; i++) {
+      await fs.write(
+        'app/globals.css',
+        css`
+          @import 'tailwindcss';
+
+          .custom-${i} {
+            color: red;
+          }
+        `,
+      )
+
+      await retryAssertion(async () => {
+        let styles = await fetchStyles(url)
+        expect(styles).toContain(candidate`flex`)
+        expect(styles).toContain(candidate`custom-${i}`)
+        expect(styles).not.toContain(candidate`custom-${i - 1}`)
+      })
+    }
+  },
+)
diff --git a/integrations/webpack/loader.test.ts b/integrations/webpack/loader.test.ts
index 9c3d870af..2bd115ab3 100644
--- a/integrations/webpack/loader.test.ts
+++ b/integrations/webpack/loader.test.ts
@@ -493,3 +493,96 @@ test(
     await fs.expectFileNotToContain('dist/b.css', ['only-a', '--color-red-500'])
   },
 )
+
+test(
+  '@tailwindcss/webpack loader rebuilds when a loader before it changes the CSS (watch)',
+  {
+    fs: {
+      'package.json': json`
+        {
+          "main": "./src/index.js",
+          "browser": "./src/index.js",
+          "dependencies": {
+            "css-loader": "^6",
+            "webpack": "^5",
+            "webpack-cli": "^5",
+            "mini-css-extract-plugin": "^2",
+            "tailwindcss": "workspace:^",
+            "@tailwindcss/webpack": "workspace:^"
+          }
+        }
+      `,
+      'webpack.config.js': js`
+        let MiniCssExtractPlugin = require('mini-css-extract-plugin')
+        let path = require('node:path')
+
+        module.exports = {
+          output: {
+            clean: true,
+          },
+          plugins: [new MiniCssExtractPlugin()],
+          module: {
+            rules: [
+              {
+                test: /.css$/i,
+                use: [
+                  MiniCssExtractPlugin.loader,
+                  'css-loader',
+                  '@tailwindcss/webpack',
+                  path.resolve(__dirname, 'partial-loader.js'),
+                ],
+              },
+            ],
+          },
+        }
+      `,
+      // Inlines a partial like Sass does, so the CSS changes while the input
+      // file on disk doesn't
+      'partial-loader.js': js`
+        let fs = require('node:fs')
+        let path = require('node:path')
+
+        module.exports = function (source) {
+          let partial = path.resolve(__dirname, 'src/partial.css')
+          this.addDependency(partial)
+          return source + fs.readFileSync(partial, 'utf8')
+        }
+      `,
+      'src/index.js': js`import './index.css'`,
+      'src/index.html': html`
+        <div class="flex"></div>
+      `,
+      'src/index.css': css`
+        @import 'tailwindcss/theme';
+        @import 'tailwindcss/utilities';
+      `,
+      'src/partial.css': css`
+        .partial-1 {
+          color: red;
+        }
+      `,
+    },
+  },
+  async ({ fs, spawn, exec }) => {
+    // Generate the initial build so output CSS files exist on disk
+    await exec('pnpm webpack --mode=development')
+
+    let process = await spawn('pnpm webpack --mode=development --watch')
+    await process.onStdout((m) => m.includes('compiled successfully in'))
+
+    await fs.expectFileToContain('dist/main.css', ['.flex', '.partial-1'])
+
+    await fs.write(
+      'src/partial.css',
+      css`
+        .partial-2 {
+          color: red;
+        }
+      `,
+    )
+    await process.onStdout((m) => m.includes('compiled successfully in'))
+
+    await fs.expectFileToContain('dist/main.css', ['.flex', '.partial-2'])
+    await fs.expectFileNotToContain('dist/main.css', ['.partial-1'])
+  },
+)
diff --git a/packages/@tailwindcss-turbopack/src/index.ts b/packages/@tailwindcss-turbopack/src/index.ts
index a70f28871..2320cb574 100644
--- a/packages/@tailwindcss-turbopack/src/index.ts
+++ b/packages/@tailwindcss-turbopack/src/index.ts
@@ -32,6 +32,7 @@ export interface LoaderOptions {
 
 interface CacheEntry {
   mtimes: Map<string, number>
+  inputCss: string
   compiler: null | Awaited<ReturnType<typeof compile>>
   scanner: null | Scanner
   candidates: Set<string>
@@ -49,6 +50,7 @@ function getContextFromCache(resourceId: string, opts: LoaderOptions): CacheEntr
   if (cache.has(key)) return cache.get(key)!
   let entry: CacheEntry = {
     mtimes: new Map<string, number>(),
+    inputCss: '',
     compiler: null,
     scanner: null,
     candidates: new Set<string>(),
@@ -162,6 +164,17 @@ export default async function tailwindLoader(
         rebuildStrategy = 'full'
         context.mtimes.set(file, changedTime)
       }
+
+      // The mtimes above only track files on disk, but the source we receive can
+      // differ from what's on disk: Turbopack can run the loader with the
+      // previous version of the input file after its mtime already changed, and
+      // loaders that run before this one (e.g. Sass) can change the source
+      // without changing any tracked file. So also rebuild when the source
+      // differs from the last build.
+      if (context.inputCss !== source) {
+        rebuildStrategy = 'full'
+        context.inputCss = source
+      }
     }
     DEBUG && I.end('Register full rebuild paths')
 
diff --git a/packages/@tailwindcss-webpack/src/index.ts b/packages/@tailwindcss-webpack/src/index.ts
index 6d02f1f0b..c714230a9 100644
--- a/packages/@tailwindcss-webpack/src/index.ts
+++ b/packages/@tailwindcss-webpack/src/index.ts
@@ -32,6 +32,7 @@ export interface LoaderOptions {
 
 interface CacheEntry {
   mtimes: Map<string, number>
+  inputCss: string
   compiler: null | Awaited<ReturnType<typeof compile>>
   scanner: null | Scanner
   candidates: Set<string>
@@ -49,6 +50,7 @@ function getContextFromCache(resourceId: string, opts: LoaderOptions): CacheEntr
   if (cache.has(key)) return cache.get(key)!
   let entry: CacheEntry = {
     mtimes: new Map<string, number>(),
+    inputCss: '',
     compiler: null,
     scanner: null,
     candidates: new Set<string>(),
@@ -162,6 +164,17 @@ export default async function tailwindLoader(
         rebuildStrategy = 'full'
         context.mtimes.set(file, changedTime)
       }
+
+      // The mtimes above only track files on disk, but the source we receive can
+      // differ from what's on disk: Turbopack can run the loader with the
+      // previous version of the input file after its mtime already changed, and
+      // loaders that run before this one (e.g. Sass) can change the source
+      // without changing any tracked file. So also rebuild when the source
+      // differs from the last build.
+      if (context.inputCss !== source) {
+        rebuildStrategy = 'full'
+        context.inputCss = source
+      }
     }
     DEBUG && I.end('Register full rebuild paths')