diff --git a/implementors/node/child_process.d.ts b/implementors/node/child_process.d.ts
index d63b290..73bbc68 100644
--- a/implementors/node/child_process.d.ts
+++ b/implementors/node/child_process.d.ts
@@ -1,6 +1,7 @@
 export interface SpawnTestOptions {
   cwd?: string;
   stdout?: 'pipe' | 'inherit';
+  worker?: boolean;
 }
 
 export interface SpawnTestResult {
diff --git a/implementors/node/child_process.js b/implementors/node/child_process.js
index d43e85b..641cc91 100644
--- a/implementors/node/child_process.js
+++ b/implementors/node/child_process.js
@@ -7,6 +7,10 @@ import { pathToFileURL } from 'node:url';
 // into one --import keeps the child's command line short.
 const HARNESS_MODULE_PATH = path.join(import.meta.dirname, 'harness.js');
 
+// Entry point used for `worker: true`: it takes the test file as an argument
+// and runs it on a worker thread of the child rather than on its main thread.
+const WORKER_ENTRY_PATH = path.join(import.meta.dirname, 'worker-entry.js');
+
 // Exit codes that signify the runtime aborted (rather than exiting cleanly with
 // a non-zero status). On POSIX an abort surfaces as a fatal signal; on Windows
 // as one of a small set of exit codes. Mirrors Node.js's
@@ -22,12 +26,19 @@ const ABORT_EXIT_CODES = [132, 133, 134, 139, 0xc0000409, 0xc000001d];
  *
  * @param {string} filePath - Path to the JS/MJS file to execute. Resolved
  *   against `options.cwd` if relative.
- * @param {{ cwd?: string, stdout?: 'pipe' | 'inherit' }} [options]
+ * @param {{ cwd?: string, stdout?: 'pipe' | 'inherit', worker?: boolean }} [options]
  *   - `cwd`: working directory for the child; defaults to `process.cwd()`.
  *   - `stdout`: `'pipe'` (default) captures the child's stdout into the result;
  *     `'inherit'` streams it straight to the terminal as the child runs (so the
  *     output of a slow or hanging test is visible immediately) and leaves the
  *     returned `stdout` empty. stderr is always captured for diagnostics.
+ *   - `worker`: run the file in a worker thread of the child instead of on its
+ *     main thread, giving it a secondary Node-API environment. The result still
+ *     describes the host process, which is the point: native output from the
+ *     worker's environment (a printf from a finalizer or an instance-data delete
+ *     hook) goes to the process's stdout, not to the worker's JS-level stream.
+ *     Gate such a test in the parent file: `skipTest()` inside a worker ends
+ *     that thread with code 0, which the caller cannot tell from a pass.
  * @returns {Promise<{ status: number | null, aborted: boolean, stdout: string, stderr: string }>}
  */
 export const spawnTest = (filePath, options = {}) => {
@@ -35,11 +46,13 @@ export const spawnTest = (filePath, options = {}) => {
   // without it.
   // pathToFileURL handles Windows drive letters and backslashes; a bare
   // 'file://' + path is malformed there (e.g. file://C:\...).
+  // In worker mode the child runs worker-entry.js, which takes the test file as
+  // its argument and starts it on a worker thread.
   const args = [
     '--expose-gc',
     '--import',
     pathToFileURL(HARNESS_MODULE_PATH).href,
-    filePath,
+    ...(options.worker ? [WORKER_ENTRY_PATH, filePath] : [filePath]),
   ];
 
   // spawn (not spawnSync) so a hung child doesn't block the event loop and the
diff --git a/implementors/node/features.js b/implementors/node/features.js
index fec2137..6bd388f 100644
--- a/implementors/node/features.js
+++ b/implementors/node/features.js
@@ -24,6 +24,13 @@ globalThis.runtimeFeatures = {
   // and need not provide a spawnTest implementation.
   spawn: true,
 
+  // Node.js can run a test file in a worker thread, giving it a secondary
+  // Node-API environment, so spawnTest accepts `{ worker: true }`. Declared
+  // separately from `spawn` because the two capabilities are independent: a
+  // browser has workers but no subprocesses. Runtimes with neither set both to
+  // false.
+  worker: true,
+
   // napi_create_dataview accepts a SharedArrayBuffer-backed buffer only since
   // Node.js v24.13.1 and v25.4.0 (nodejs/node#60473). It was not backported to
   // v20.x or v22.x, where such calls fail with "invalid argument".
diff --git a/implementors/node/worker-entry.js b/implementors/node/worker-entry.js
new file mode 100644
index 0000000..9fd41a9
--- /dev/null
+++ b/implementors/node/worker-entry.js
@@ -0,0 +1,18 @@
+import path from 'node:path';
+import { pathToFileURL } from 'node:url';
+import { Worker } from 'node:worker_threads';
+
+// Entry point for spawnTest(file, { worker: true }): boots the test file in a
+// worker thread, giving it a secondary Node-API environment inside this process.
+//
+// The worker inherits this process's execArgv, so the harness --import applies
+// there too and the test file sees the same globals as on the main thread.
+//
+// No 'error' handler is installed on purpose: an unhandled worker error is
+// re-thrown on this thread, so a failing test file still exits the process
+// non-zero with its stack on stderr, exactly as it would on the main thread.
+const [filePath] = process.argv.slice(2);
+
+// Worker resolves a relative specifier against the cwd, but a bare filename
+// (no leading './') would read as a package specifier - so make it absolute.
+new Worker(pathToFileURL(path.resolve(filePath)));
diff --git a/tests/harness/spawn-test-worker.js b/tests/harness/spawn-test-worker.js
new file mode 100644
index 0000000..06c1128
--- /dev/null
+++ b/tests/harness/spawn-test-worker.js
@@ -0,0 +1,50 @@
+'use strict';
+
+// Running a test file in a secondary environment (a worker) is an optional
+// harness capability, declared separately from `spawn` because a runtime can
+// have one without the other: a browser has workers but no subprocesses.
+//
+// The Node harness delivers it as an option on spawnTest rather than as its own
+// global, because observing a secondary environment's *native* output requires
+// capturing the stdout of the process hosting it - a worker's own piped stdout
+// carries JS-level writes only, so a printf from an addon bypasses it.
+assert.strictEqual(
+  typeof runtimeFeatures.worker,
+  'boolean',
+  'Expected runtimeFeatures.worker to be a boolean',
+);
+if (!runtimeFeatures.spawn || !runtimeFeatures.worker) {
+  skipTest();
+}
+
+// That the file ran in a secondary environment rather than the main one is not
+// observable from portable ECMAScript; proving that needs per-environment
+// Node-API state (see the test_instance_data suite). What is pinned here is the
+// plumbing: the file runs, harness globals reach it, and a failure inside the
+// worker still surfaces as a non-zero status with its stderr intact instead of
+// being swallowed by the host thread.
+{
+  const result = await spawnTest('spawn-test-ok-child.mjs', { worker: true });
+  assert.strictEqual(result.status, 0, `ok child exited with status ${result.status}; stderr:\n${result.stderr}`);
+  assert.strictEqual(result.aborted, false);
+  assert.strictEqual(result.stderr, '');
+}
+
+{
+  const result = await spawnTest('spawn-test-fail-child.mjs', { worker: true });
+  assert.notStrictEqual(result.status, 0, 'fail child should exit non-zero');
+  assert.strictEqual(result.aborted, false);
+  if (!result.stderr.includes('spawn-test-fail-marker')) {
+    throw new Error(`Expected stderr to include the failure marker, got:\n${result.stderr}`);
+  }
+}
+
+// cwd still applies in worker mode: the test file is resolved against it, so an
+// unresolvable filename must fail loudly rather than pass as an empty worker.
+{
+  const result = await spawnTest('spawn-test-ok-child.mjs', { worker: true, cwd: '..' });
+  assert.notStrictEqual(result.status, 0, 'expected cwd ".." to make the child filename unresolvable');
+  if (!result.stderr.includes('spawn-test-ok-child.mjs')) {
+    throw new Error(`Expected stderr to reference the unresolved child filename, got:\n${result.stderr}`);
+  }
+}