diff --git a/__snapshots__/tsnapi/index.snapshot.d.ts b/__snapshots__/tsnapi/index.snapshot.d.ts
index c9d35b2ad..d39844f54 100644
--- a/__snapshots__/tsnapi/index.snapshot.d.ts
+++ b/__snapshots__/tsnapi/index.snapshot.d.ts
@@ -54,6 +54,7 @@ export interface ExportsOptions {
     isPublish: boolean;
   }) => Awaitable<Record<string, any>>);
   inlinedDependencies?: boolean;
+  extensions?: boolean;
   bin?: boolean | string | Record<string, string>;
 }
 export interface InlineConfig extends UserConfig {
diff --git a/src/features/pkg/exports.test.ts b/src/features/pkg/exports.test.ts
index c9841dd11..d4903adc1 100644
--- a/src/features/pkg/exports.test.ts
+++ b/src/features/pkg/exports.test.ts
@@ -866,6 +866,160 @@ describe('generateExports', () => {
     })
   })
 
+  test('extensions adds .js to subpath export keys', async ({ expect }) => {
+    const results = generateExports(
+      {
+        es: [genChunk('index.js'), genChunk('foo.js'), genChunk('bar.js')],
+      },
+      {
+        exports: { extensions: true },
+      },
+    )
+    await expect(results).resolves.toMatchInlineSnapshot(`
+      {
+        "bin": undefined,
+        "exports": {
+          ".": "./index.js",
+          "./bar.js": "./bar.js",
+          "./foo.js": "./foo.js",
+          "./package.json": "./package.json",
+        },
+        "inlinedDependencies": undefined,
+        "main": undefined,
+        "module": undefined,
+        "publishExports": undefined,
+        "types": undefined,
+      }
+    `)
+  })
+
+  test('extensions with directory index entries', async ({ expect }) => {
+    const results = generateExports(
+      {
+        es: [genChunk('index.js'), genChunk('foo/index.js')],
+      },
+      {
+        exports: { extensions: true },
+      },
+    )
+    await expect(results).resolves.toMatchInlineSnapshot(`
+      {
+        "bin": undefined,
+        "exports": {
+          ".": "./index.js",
+          "./foo.js": "./foo/index.js",
+          "./package.json": "./package.json",
+        },
+        "inlinedDependencies": undefined,
+        "main": undefined,
+        "module": undefined,
+        "publishExports": undefined,
+        "types": undefined,
+      }
+    `)
+  })
+
+  test('extensions with dual formats', async ({ expect }) => {
+    const results = generateExports(
+      {
+        es: [genChunk('index.js'), genChunk('utils.js')],
+        cjs: [genChunk('index.cjs'), genChunk('utils.cjs')],
+      },
+      {
+        exports: { extensions: true },
+      },
+    )
+    await expect(results).resolves.toMatchInlineSnapshot(`
+      {
+        "bin": undefined,
+        "exports": {
+          ".": {
+            "import": "./index.js",
+            "require": "./index.cjs",
+          },
+          "./package.json": "./package.json",
+          "./utils.js": {
+            "import": "./utils.js",
+            "require": "./utils.cjs",
+          },
+        },
+        "inlinedDependencies": undefined,
+        "main": "./index.cjs",
+        "module": "./index.js",
+        "publishExports": undefined,
+        "types": undefined,
+      }
+    `)
+  })
+
+  test('extensions does not affect root export', async ({ expect }) => {
+    const results = generateExports(
+      {
+        es: [genChunk('main.js')],
+      },
+      {
+        exports: { extensions: true },
+      },
+    )
+    await expect(results).resolves.toMatchInlineSnapshot(`
+      {
+        "bin": undefined,
+        "exports": {
+          ".": "./main.js",
+          "./package.json": "./package.json",
+        },
+        "inlinedDependencies": undefined,
+        "main": undefined,
+        "module": undefined,
+        "publishExports": undefined,
+        "types": undefined,
+      }
+    `)
+  })
+
+  test('extensions with devExports', async ({ expect }) => {
+    const results = await generateExports(
+      {
+        es: [genChunk('index.js'), genChunk('utils.js')],
+        cjs: [genChunk('index.cjs'), genChunk('utils.cjs')],
+      },
+      {
+        exports: { extensions: true, devExports: '@my-org/source' },
+      },
+    )
+    // key order matters
+    expect(JSON.stringify(results, undefined, 2)).toMatchInlineSnapshot(`
+      "{
+        "main": "./index.cjs",
+        "module": "./index.js",
+        "exports": {
+          ".": {
+            "@my-org/source": "./SRC/index.js",
+            "import": "./index.js",
+            "require": "./index.cjs"
+          },
+          "./utils.js": {
+            "@my-org/source": "./SRC/utils.js",
+            "import": "./utils.js",
+            "require": "./utils.cjs"
+          },
+          "./package.json": "./package.json"
+        },
+        "publishExports": {
+          ".": {
+            "import": "./index.js",
+            "require": "./index.cjs"
+          },
+          "./utils.js": {
+            "import": "./utils.js",
+            "require": "./utils.cjs"
+          },
+          "./package.json": "./package.json"
+        }
+      }"
+    `)
+  })
+
   test('generate css publish exports', async ({ expect }) => {
     const results = generateExports(
       { es: [genChunk('index.js'), genAsset('style.css')] },
diff --git a/src/features/pkg/exports.ts b/src/features/pkg/exports.ts
index 08da58600..a3a94cbc7 100644
--- a/src/features/pkg/exports.ts
+++ b/src/features/pkg/exports.ts
@@ -96,6 +96,19 @@ export interface ExportsOptions {
    */
   inlinedDependencies?: boolean
 
+  /**
+   * Add file extensions to subpath export keys.
+   *
+   * When enabled, all subpath exports (except the root `"."`) will include
+   * a `.js` extension in the key (e.g., `"./utils.js"` instead of `"./utils"`).
+   *
+   * This follows the Node.js recommendation for subpath exports:
+   * @see {@link https://nodejs.org/api/packages.html#extensions-in-subpaths}
+   *
+   * @default false
+   */
+  extensions?: boolean
+
   /**
    * Auto-generate the `bin` field in package.json.
    *
@@ -185,6 +198,7 @@ export async function generateExports(
       exclude,
       customExports,
       legacy,
+      extensions,
       inlinedDependencies: emitInlinedDeps = true,
       bin,
     },
@@ -268,6 +282,10 @@ export async function generateExports(
         name = `./${name}`
       }
 
+      if (extensions && name !== '.') {
+        name = `${name}.js`
+      }
+
       let subExport = exportsMap.get(name)
       if (!subExport) {
         subExport = {}