diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6f3b0b97718c..682b66628eb2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- Ensure validation of `source(…)` happens relative to the file it is in ([#19274](https://github.com/tailwindlabs/tailwindcss/pull/19274))
+
 ### Added
 
 - _Experimental_: Add `@container-size` utility ([#18901](https://github.com/tailwindlabs/tailwindcss/pull/18901))
diff --git a/integrations/cli/index.test.ts b/integrations/cli/index.test.ts
index be48a2ac4898..9b666f4ec313 100644
--- a/integrations/cli/index.test.ts
+++ b/integrations/cli/index.test.ts
@@ -1332,6 +1332,65 @@ test(
   },
 )
 
+test(
+  'source(…) and `@source` are relative to the file they are in',
+  {
+    fs: {
+      'package.json': json`
+        {
+          "dependencies": {
+            "tailwindcss": "workspace:^",
+            "@tailwindcss/cli": "workspace:^"
+          }
+        }
+      `,
+      'index.css': css` @import './project-a/src/index.css'; `,
+
+      'project-a/src/index.css': css`
+        /* Run auto-content detection in ../../project-b */
+        @import 'tailwindcss/utilities' source('../../project-b');
+
+        /* Explicitly using node_modules in the @source allows git ignored folders */
+        @source '../../project-c';
+      `,
+
+      // Project A is the current folder, but we explicitly configured
+      // `source(project-b)`, therefore project-a should not be included in
+      // the output.
+      'project-a/src/index.html': html`
+        <div
+          class="content-['SHOULD-NOT-EXIST-IN-OUTPUT'] content-['project-a/src/index.html']"
+        ></div>
+      `,
+
+      // Project B is the configured `source(…)`, therefore auto source
+      // detection should include known extensions and folders in the output.
+      'project-b/src/index.html': html`
+        <div
+          class="content-['project-b/src/index.html']"
+        ></div>
+      `,
+
+      // Project C should apply auto source detection, therefore known
+      // extensions and folders should be included in the output.
+      'project-c/src/index.html': html`
+        <div
+          class="content-['project-c/src/index.html']"
+        ></div>
+      `,
+    },
+  },
+  async ({ fs, exec, spawn, root, expect }) => {
+    await exec('pnpm tailwindcss --input ./index.css --output dist/out.css', { cwd: root })
+
+    let content = await fs.dumpFiles('./dist/*.css')
+
+    expect(content).not.toContain(candidate`content-['project-a/src/index.html']`)
+    expect(content).toContain(candidate`content-['project-b/src/index.html']`)
+    expect(content).toContain(candidate`content-['project-c/src/index.html']`)
+  },
+)
+
 test(
   'auto source detection disabled',
   {
diff --git a/integrations/postcss/index.test.ts b/integrations/postcss/index.test.ts
index dd07fd90ebbd..08ea852f950c 100644
--- a/integrations/postcss/index.test.ts
+++ b/integrations/postcss/index.test.ts
@@ -825,3 +825,73 @@ test(
     })
   },
 )
+
+test(
+  'source(…) and `@source` are relative to the file they are in',
+  {
+    fs: {
+      'package.json': json`
+        {
+          "dependencies": {
+            "postcss": "^8",
+            "postcss-cli": "^10",
+            "tailwindcss": "workspace:^",
+            "@tailwindcss/postcss": "workspace:^"
+          }
+        }
+      `,
+
+      'postcss.config.js': js`
+        module.exports = {
+          plugins: {
+            '@tailwindcss/postcss': {},
+          },
+        }
+      `,
+
+      'index.css': css` @import './project-a/src/index.css'; `,
+
+      'project-a/src/index.css': css`
+        /* Run auto-content detection in ../../project-b */
+        @import 'tailwindcss/utilities' source('../../project-b');
+
+        /* Explicitly using node_modules in the @source allows git ignored folders */
+        @source '../../project-c';
+      `,
+
+      // Project A is the current folder, but we explicitly configured
+      // `source(project-b)`, therefore project-a should not be included in
+      // the output.
+      'project-a/src/index.html': html`
+        <div
+          class="content-['SHOULD-NOT-EXIST-IN-OUTPUT'] content-['project-a/src/index.html']"
+        ></div>
+      `,
+
+      // Project B is the configured `source(…)`, therefore auto source
+      // detection should include known extensions and folders in the output.
+      'project-b/src/index.html': html`
+        <div
+          class="content-['project-b/src/index.html']"
+        ></div>
+      `,
+
+      // Project C should apply auto source detection, therefore known
+      // extensions and folders should be included in the output.
+      'project-c/src/index.html': html`
+        <div
+          class="content-['project-c/src/index.html']"
+        ></div>
+      `,
+    },
+  },
+  async ({ fs, exec, root, expect }) => {
+    await exec('pnpm postcss ./index.css --output dist/out.css', { cwd: root })
+
+    let content = await fs.dumpFiles('./dist/*.css')
+
+    expect(content).not.toContain(candidate`content-['project-a/src/index.html']`)
+    expect(content).toContain(candidate`content-['project-b/src/index.html']`)
+    expect(content).toContain(candidate`content-['project-c/src/index.html']`)
+  },
+)
diff --git a/integrations/vite/index.test.ts b/integrations/vite/index.test.ts
index ead71b47296f..91570a6fcc2d 100644
--- a/integrations/vite/index.test.ts
+++ b/integrations/vite/index.test.ts
@@ -730,6 +730,86 @@ describe.each(['postcss', 'lightningcss'])('%s', (transformer) => {
       expect(files).toHaveLength(0)
     },
   )
+
+  test(
+    'source(…) and `@source` are relative to the file they are in',
+    {
+      fs: {
+        'package.json': json`
+          {
+            "type": "module",
+            "dependencies": {
+              "@tailwindcss/vite": "workspace:^",
+              "tailwindcss": "workspace:^"
+            },
+            "devDependencies": {
+              ${transformer === 'lightningcss' ? `"lightningcss": "^1",` : ''}
+              "vite": "^7"
+            }
+          }
+        `,
+        'vite.config.ts': ts`
+          import tailwindcss from '@tailwindcss/vite'
+          import { defineConfig } from 'vite'
+
+          export default defineConfig({
+            css: ${transformer === 'postcss' ? '{}' : "{ transformer: 'lightningcss' }"},
+            build: { cssMinify: false },
+            plugins: [tailwindcss()],
+          })
+        `,
+        'index.html': html`
+          <head>
+            <link rel="stylesheet" href="/index.css" />
+          </head>
+          <body></body>
+        `,
+        'index.css': css` @import './project-a/src/index.css'; `,
+
+        'project-a/src/index.css': css`
+          /* Run auto-content detection in ../../project-b */
+          @import 'tailwindcss/utilities' source('../../project-b');
+
+          /* Explicitly using node_modules in the @source allows git ignored folders */
+          @source '../../project-c';
+        `,
+
+        // Project A is the current folder, but we explicitly configured
+        // `source(project-b)`, therefore project-a should not be included in
+        // the output.
+        'project-a/src/index.html': html`
+          <div
+            class="content-['SHOULD-NOT-EXIST-IN-OUTPUT'] content-['project-a/src/index.html']"
+          ></div>
+        `,
+
+        // Project B is the configured `source(…)`, therefore auto source
+        // detection should include known extensions and folders in the output.
+        'project-b/src/index.html': html`
+          <div
+            class="content-['project-b/src/index.html']"
+          ></div>
+        `,
+
+        // Project C should apply auto source detection, therefore known
+        // extensions and folders should be included in the output.
+        'project-c/src/index.html': html`
+          <div
+            class="content-['project-c/src/index.html']"
+          ></div>
+        `,
+      },
+    },
+    async ({ fs, exec, spawn, root, expect }) => {
+      await exec('pnpm vite build', { cwd: root })
+
+      let content = await fs.dumpFiles('./dist/assets/*.css')
+
+      expect(content).not.toContain(candidate`content-['project-a/src/index.html']`)
+      expect(content).toContain(candidate`content-['project-b/src/index.html']`)
+      expect(content).toContain(candidate`content-['project-c/src/index.html']`)
+    },
+  )
 })
 
 test(
diff --git a/packages/@tailwindcss-node/src/compile.ts b/packages/@tailwindcss-node/src/compile.ts
index 5ce6299af6a2..ac7cc6f15211 100644
--- a/packages/@tailwindcss-node/src/compile.ts
+++ b/packages/@tailwindcss-node/src/compile.ts
@@ -63,10 +63,9 @@ function createCompileOptions({
   }
 }
 
-async function ensureSourceDetectionRootExists(
-  compiler: { root: Awaited<ReturnType<typeof compile>>['root'] },
-  base: string,
-) {
+async function ensureSourceDetectionRootExists(compiler: {
+  root: Awaited<ReturnType<typeof compile>>['root']
+}) {
   // Verify if the `source(…)` path exists (until the glob pattern starts)
   if (compiler.root && compiler.root !== 'none') {
     let globSymbols = /[*{]/
@@ -80,25 +79,27 @@ async function ensureSourceDetectionRootExists(
     }
 
     let exists = await fsPromises
-      .stat(path.resolve(base, basePath.join('/')))
+      .stat(path.resolve(compiler.root.base, basePath.join('/')))
       .then((stat) => stat.isDirectory())
       .catch(() => false)
 
     if (!exists) {
-      throw new Error(`The \`source(${compiler.root.pattern})\` does not exist`)
+      throw new Error(
+        `The \`source(${compiler.root.pattern})\` does not exist or is not a directory.`,
+      )
     }
   }
 }
 
 export async function compileAst(ast: AstNode[], options: CompileOptions) {
   let compiler = await _compileAst(ast, createCompileOptions(options))
-  await ensureSourceDetectionRootExists(compiler, options.base)
+  await ensureSourceDetectionRootExists(compiler)
   return compiler
 }
 
 export async function compile(css: string, options: CompileOptions) {
   let compiler = await _compile(css, createCompileOptions(options))
-  await ensureSourceDetectionRootExists(compiler, options.base)
+  await ensureSourceDetectionRootExists(compiler)
   return compiler
 }