diff --git a/cli/tests/node_compat/config.jsonc b/cli/tests/node_compat/config.jsonc
index a3b97b5100..ccc83cd3de 100644
--- a/cli/tests/node_compat/config.jsonc
+++ b/cli/tests/node_compat/config.jsonc
@@ -1,7 +1,7 @@
 {
   "nodeVersion": "18.12.1",
   "ignore": {
-    "common": ["index.js", "internet.js", "tmpdir.js"],
+    "common": ["index.js", "internet.js"],
     "fixtures": [
       "child-process-spawn-node.js",
       "echo.js",
@@ -121,7 +121,8 @@
       "fixtures.js",
       "hijackstdio.js",
       "index.mjs",
-      "internet.js"
+      "internet.js",
+      "tmpdir.js"
     ],
     "fixtures": [
       "GH-1899-output.js",
diff --git a/cli/tests/node_compat/test/common/tmpdir.js b/cli/tests/node_compat/test/common/tmpdir.js
index d3ce98e45b..dbc945c137 100644
--- a/cli/tests/node_compat/test/common/tmpdir.js
+++ b/cli/tests/node_compat/test/common/tmpdir.js
@@ -2,14 +2,14 @@
 // deno-lint-ignore-file
 
 // Copyright Joyent and Node contributors. All rights reserved. MIT license.
-// Taken from Node 16.13.0
+// Taken from Node 18.12.1
 // This file is automatically generated by "node/_tools/setup.ts". Do not modify this file manually
 
 'use strict';
 
 const fs = require('fs');
 const path = require('path');
-// const { isMainThread } = require('worker_threads');
+const { isMainThread } = require('worker_threads');
 
 function rmSync(pathname) {
   fs.rmSync(pathname, { maxRetries: 3, recursive: true, force: true });
@@ -26,8 +26,8 @@ const tmpPath = path.join(testRoot, tmpdirName);
 
 let firstRefresh = true;
 function refresh() {
-  rmSync(this.path);
-  fs.mkdirSync(this.path);
+  rmSync(tmpPath);
+  fs.mkdirSync(tmpPath);
 
   if (firstRefresh) {
     firstRefresh = false;
@@ -39,9 +39,8 @@ function refresh() {
 
 function onexit() {
   // Change directory to avoid possible EBUSY
-  // TODO(f3n67u): uncomment when `worker_thread.isMainThread` implemented
-  // if (isMainThread)
-  //   process.chdir(testRoot);
+  if (isMainThread)
+    process.chdir(testRoot);
 
   try {
     rmSync(tmpPath);
diff --git a/cli/tests/unit_node/testdata/worker_threads.mjs b/cli/tests/unit_node/testdata/worker_threads.mjs
new file mode 100644
index 0000000000..03dc462f02
--- /dev/null
+++ b/cli/tests/unit_node/testdata/worker_threads.mjs
@@ -0,0 +1,34 @@
+// Copyright 2018-2023 the Deno authors. All rights reserved. MIT license.
+import {
+  getEnvironmentData,
+  isMainThread,
+  parentPort,
+  threadId,
+  workerData,
+} from "node:worker_threads";
+import { once } from "node:events";
+
+async function message(expectedMessage) {
+  const [message] = await once(parentPort, "message");
+  if (message !== expectedMessage) {
+    console.log(`Expected the message "${expectedMessage}", but got`, message);
+    // fail test
+    parentPort.close();
+  }
+}
+
+await message("Hello, how are you my thread?");
+
+parentPort.postMessage("I'm fine!");
+
+await new Promise((resolve) => setTimeout(resolve, 100));
+
+parentPort.postMessage({
+  isMainThread,
+  threadId,
+  workerData: Array.isArray(workerData) &&
+      workerData[workerData.length - 1] instanceof MessagePort
+    ? workerData.slice(0, -1)
+    : workerData,
+  envData: [getEnvironmentData("test"), getEnvironmentData(1)],
+});
diff --git a/cli/tests/unit_node/worker_threads_test.ts b/cli/tests/unit_node/worker_threads_test.ts
index 17de7cca1e..f53b1e6927 100644
--- a/cli/tests/unit_node/worker_threads_test.ts
+++ b/cli/tests/unit_node/worker_threads_test.ts
@@ -1,7 +1,13 @@
 // Copyright 2018-2023 the Deno authors. All rights reserved. MIT license.
 
-import { assertEquals } from "../../../test_util/std/testing/asserts.ts";
-import workerThreads from "node:worker_threads";
+import {
+  assert,
+  assertEquals,
+  assertObjectMatch,
+} from "../../../test_util/std/testing/asserts.ts";
+import { fromFileUrl, relative } from "../../../test_util/std/path/mod.ts";
+import * as workerThreads from "node:worker_threads";
+import { EventEmitter, once } from "node:events";
 
 Deno.test("[node/worker_threads] BroadcastChannel is exported", () => {
   assertEquals<unknown>(workerThreads.BroadcastChannel, BroadcastChannel);
@@ -11,3 +17,178 @@ Deno.test("[node/worker_threads] MessageChannel are MessagePort are exported", (
   assertEquals<unknown>(workerThreads.MessageChannel, MessageChannel);
   assertEquals<unknown>(workerThreads.MessagePort, MessagePort);
 });
+
+Deno.test({
+  name: "[worker_threads] isMainThread",
+  fn() {
+    assertEquals(workerThreads.isMainThread, true);
+  },
+});
+
+Deno.test({
+  name: "[worker_threads] threadId",
+  fn() {
+    assertEquals(workerThreads.threadId, 0);
+  },
+});
+
+Deno.test({
+  name: "[worker_threads] resourceLimits",
+  fn() {
+    assertObjectMatch(workerThreads.resourceLimits, {});
+  },
+});
+
+Deno.test({
+  name: "[worker_threads] parentPort",
+  fn() {
+    assertEquals(workerThreads.parentPort, null);
+  },
+});
+
+Deno.test({
+  name: "[worker_threads] workerData",
+  fn() {
+    assertEquals(workerThreads.workerData, null);
+  },
+});
+
+Deno.test({
+  name: "[worker_threads] setEnvironmentData / getEnvironmentData",
+  fn() {
+    workerThreads.setEnvironmentData("test", "test");
+    assertEquals(workerThreads.getEnvironmentData("test"), "test");
+  },
+});
+
+Deno.test({
+  name: "[worker_threads] Worker threadId",
+  async fn() {
+    const worker = new workerThreads.Worker(
+      new URL("./testdata/worker_threads.mjs", import.meta.url),
+    );
+    worker.postMessage("Hello, how are you my thread?");
+    await once(worker, "message");
+    const message = await once(worker, "message");
+    assertEquals(message[0].threadId, 1);
+    worker.terminate();
+
+    const worker1 = new workerThreads.Worker(
+      new URL("./testdata/worker_threads.mjs", import.meta.url),
+    );
+    worker1.postMessage("Hello, how are you my thread?");
+    await once(worker1, "message");
+    assertEquals((await once(worker1, "message"))[0].threadId, 2);
+    worker1.terminate();
+  },
+});
+
+Deno.test({
+  name: "[worker_threads] Worker basics",
+  async fn() {
+    workerThreads.setEnvironmentData("test", "test");
+    workerThreads.setEnvironmentData(1, {
+      test: "random",
+      random: "test",
+    });
+    const { port1 } = new MessageChannel();
+    const worker = new workerThreads.Worker(
+      new URL("./testdata/worker_threads.mjs", import.meta.url),
+      {
+        workerData: ["hey", true, false, 2, port1],
+        // deno-lint-ignore no-explicit-any
+        transferList: [port1 as any],
+      },
+    );
+    worker.postMessage("Hello, how are you my thread?");
+    assertEquals((await once(worker, "message"))[0], "I'm fine!");
+    const data = (await once(worker, "message"))[0];
+    // data.threadId can be 1 when this test is runned individually
+    if (data.threadId === 1) data.threadId = 3;
+    assertObjectMatch(data, {
+      isMainThread: false,
+      threadId: 3,
+      workerData: ["hey", true, false, 2],
+      envData: ["test", { test: "random", random: "test" }],
+    });
+    worker.terminate();
+  },
+  sanitizeResources: false,
+});
+
+Deno.test({
+  name: "[worker_threads] Worker eval",
+  async fn() {
+    const worker = new workerThreads.Worker(
+      `
+      import { parentPort } from "node:worker_threads";
+      parentPort.postMessage("It works!");
+      `,
+      {
+        eval: true,
+      },
+    );
+    assertEquals((await once(worker, "message"))[0], "It works!");
+    worker.terminate();
+  },
+});
+
+Deno.test({
+  name: "[worker_threads] inheritences",
+  async fn() {
+    const worker = new workerThreads.Worker(
+      `
+      import { EventEmitter } from "node:events";
+      import { parentPort } from "node:worker_threads";
+      parentPort.postMessage(parentPort instanceof EventTarget);
+      await new Promise(resolve => setTimeout(resolve, 100));
+      parentPort.postMessage(parentPort instanceof EventEmitter);
+      `,
+      {
+        eval: true,
+      },
+    );
+    assertEquals((await once(worker, "message"))[0], true);
+    assertEquals((await once(worker, "message"))[0], false);
+    assert(worker instanceof EventEmitter);
+    assert(!(worker instanceof EventTarget));
+    worker.terminate();
+  },
+});
+
+Deno.test({
+  name: "[worker_threads] Worker workerData",
+  async fn() {
+    const worker = new workerThreads.Worker(
+      new URL("./testdata/worker_threads.mjs", import.meta.url),
+      {
+        workerData: null,
+      },
+    );
+    worker.postMessage("Hello, how are you my thread?");
+    await once(worker, "message");
+    assertEquals((await once(worker, "message"))[0].workerData, null);
+    worker.terminate();
+
+    const worker1 = new workerThreads.Worker(
+      new URL("./testdata/worker_threads.mjs", import.meta.url),
+    );
+    worker1.postMessage("Hello, how are you my thread?");
+    await once(worker1, "message");
+    assertEquals((await once(worker1, "message"))[0].workerData, undefined);
+    worker1.terminate();
+  },
+});
+
+Deno.test({
+  name: "[worker_threads] Worker with relative path",
+  async fn() {
+    const worker = new workerThreads.Worker(relative(
+      Deno.cwd(),
+      fromFileUrl(new URL("./testdata/worker_threads.mjs", import.meta.url)),
+    ));
+    worker.postMessage("Hello, how are you my thread?");
+    assertEquals((await once(worker, "message"))[0], "I'm fine!");
+    worker.terminate();
+  },
+});
diff --git a/ext/node/polyfills/02_init.js b/ext/node/polyfills/02_init.js
index dc8955d834..a2fba8c0c5 100644
--- a/ext/node/polyfills/02_init.js
+++ b/ext/node/polyfills/02_init.js
@@ -46,6 +46,7 @@ function initialize(
   // FIXME(bartlomieju): not nice to depend on `Deno` namespace here
   // but it's the only way to get `args` and `version` and this point.
   internals.__bootstrapNodeProcess(argv0, Deno.args, Deno.version);
+  internals.__initWorkerThreads();
   // `Deno[Deno.internal].requireImpl` will be unreachable after this line.
   delete internals.requireImpl;
 }
diff --git a/ext/node/polyfills/worker_threads.ts b/ext/node/polyfills/worker_threads.ts
index 2c13e4bc8d..8005506bb6 100644
--- a/ext/node/polyfills/worker_threads.ts
+++ b/ext/node/polyfills/worker_threads.ts
@@ -3,11 +3,11 @@
 
 import { resolve, toFileUrl } from "ext:deno_node/path.ts";
 import { notImplemented } from "ext:deno_node/_utils.ts";
-import { EventEmitter } from "ext:deno_node/events.ts";
+import { EventEmitter, once } from "ext:deno_node/events.ts";
 import { BroadcastChannel } from "ext:deno_broadcast_channel/01_broadcast_channel.js";
 import { MessageChannel, MessagePort } from "ext:deno_web/13_message_port.js";
 
-const environmentData = new Map();
+let environmentData = new Map();
 let threads = 0;
 
 export interface WorkerOptions {
@@ -48,13 +48,18 @@ class _Worker extends EventEmitter {
   postMessage: Worker["postMessage"];
 
   constructor(specifier: URL | string, options?: WorkerOptions) {
-    notImplemented("Worker");
     super();
     if (options?.eval === true) {
       specifier = `data:text/javascript,${specifier}`;
     } else if (typeof specifier === "string") {
-      // @ts-ignore This API is temporarily disabled
-      specifier = toFileUrl(resolve(specifier));
+      specifier = resolve(specifier);
+      if (!specifier.toString().endsWith(".mjs")) {
+        const cwdFileUrl = toFileUrl(Deno.cwd());
+        specifier =
+          `data:text/javascript,(async function() {const { createRequire } = await import("node:module");const require = createRequire("${cwdFileUrl}");require("${specifier}");})();`;
+      } else {
+        specifier = toFileUrl(specifier);
+      }
     }
     const handle = this[kHandle] = new Worker(
       specifier,
@@ -95,20 +100,11 @@ class _Worker extends EventEmitter {
   readonly performance = globalThis.performance;
 }
 
-export const isMainThread =
-  // deno-lint-ignore no-explicit-any
-  (globalThis as any).name !== PRIVATE_WORKER_THREAD_NAME;
+export let isMainThread;
+export let resourceLimits;
 
-// fake resourceLimits
-export const resourceLimits = isMainThread ? {} : {
-  maxYoungGenerationSizeMb: 48,
-  maxOldGenerationSizeMb: 2048,
-  codeRangeSizeMb: 0,
-  stackSizeMb: 4,
-};
-
-const threadId = 0;
-const workerData: unknown = null;
+let threadId = 0;
+let workerData: unknown = null;
 
 // Like https://github.com/nodejs/node/blob/48655e17e1d84ba5021d7a94b4b88823f7c9c6cf/lib/internal/event_target.js#L611
 interface NodeEventTarget extends
@@ -131,74 +127,100 @@ interface NodeEventTarget extends
 type ParentPort = typeof self & NodeEventTarget;
 
 // deno-lint-ignore no-explicit-any
-const parentPort: ParentPort = null as any;
+let parentPort: ParentPort = null as any;
 
-/*
-if (!isMainThread) {
-  // deno-lint-ignore no-explicit-any
-  delete (globalThis as any).name;
-  // deno-lint-ignore no-explicit-any
-  const listeners = new WeakMap<(...args: any[]) => void, (ev: any) => any>();
-
-  parentPort = self as ParentPort;
-  parentPort.off = parentPort.removeListener = function (
-    this: ParentPort,
-    name,
-    listener,
-  ) {
-    this.removeEventListener(name, listeners.get(listener)!);
-    listeners.delete(listener);
-    return this;
-  };
-  parentPort.on = parentPort.addListener = function (
-    this: ParentPort,
-    name,
-    listener,
-  ) {
+globalThis.__bootstrap.internals.__initWorkerThreads = () => {
+  isMainThread =
     // deno-lint-ignore no-explicit-any
-    const _listener = (ev: any) => listener(ev.data);
-    listeners.set(listener, _listener);
-    this.addEventListener(name, _listener);
-    return this;
+    (globalThis as any).name !== PRIVATE_WORKER_THREAD_NAME;
+
+  defaultExport.isMainThread = isMainThread;
+  // fake resourceLimits
+  resourceLimits = isMainThread ? {} : {
+    maxYoungGenerationSizeMb: 48,
+    maxOldGenerationSizeMb: 2048,
+    codeRangeSizeMb: 0,
+    stackSizeMb: 4,
   };
-  parentPort.once = function (this: ParentPort, name, listener) {
+  defaultExport.resourceLimits = resourceLimits;
+
+  if (!isMainThread) {
     // deno-lint-ignore no-explicit-any
-    const _listener = (ev: any) => listener(ev.data);
-    listeners.set(listener, _listener);
-    this.addEventListener(name, _listener);
-    return this;
-  };
+    delete (globalThis as any).name;
+    // deno-lint-ignore no-explicit-any
+    const listeners = new WeakMap<(...args: any[]) => void, (ev: any) => any>();
 
-  // mocks
-  parentPort.setMaxListeners = () => {};
-  parentPort.getMaxListeners = () => Infinity;
-  parentPort.eventNames = () => [""];
-  parentPort.listenerCount = () => 0;
+    parentPort = self as ParentPort;
 
-  parentPort.emit = () => notImplemented("parentPort.emit");
-  parentPort.removeAllListeners = () =>
-    notImplemented("parentPort.removeAllListeners");
+    const initPromise = once(
+      parentPort,
+      "message",
+    ).then((result) => {
+      // TODO(kt3k): The below values are set asynchronously
+      // using the first message from the parent.
+      // This should be done synchronously.
+      threadId = result[0].data.threadId;
+      workerData = result[0].data.workerData;
+      environmentData = result[0].data.environmentData;
 
-  // Receive startup message
-  [{ threadId, workerData, environmentData }] = await once(
-    parentPort,
-    "message",
-  );
+      defaultExport.threadId = threadId;
+      defaultExport.workerData = workerData;
+    });
 
-  // alias
-  parentPort.addEventListener("offline", () => {
-    parentPort.emit("close");
-  });
-}
-*/
+    parentPort.off = parentPort.removeListener = function (
+      this: ParentPort,
+      name,
+      listener,
+    ) {
+      this.removeEventListener(name, listeners.get(listener)!);
+      listeners.delete(listener);
+      return this;
+    };
+    parentPort.on = parentPort.addListener = function (
+      this: ParentPort,
+      name,
+      listener,
+    ) {
+      initPromise.then(() => {
+        // deno-lint-ignore no-explicit-any
+        const _listener = (ev: any) => listener(ev.data);
+        listeners.set(listener, _listener);
+        this.addEventListener(name, _listener);
+      });
+      return this;
+    };
+
+    parentPort.once = function (this: ParentPort, name, listener) {
+      initPromise.then(() => {
+        // deno-lint-ignore no-explicit-any
+        const _listener = (ev: any) => listener(ev.data);
+        listeners.set(listener, _listener);
+        this.addEventListener(name, _listener);
+      });
+      return this;
+    };
+
+    // mocks
+    parentPort.setMaxListeners = () => {};
+    parentPort.getMaxListeners = () => Infinity;
+    parentPort.eventNames = () => [""];
+    parentPort.listenerCount = () => 0;
+
+    parentPort.emit = () => notImplemented("parentPort.emit");
+    parentPort.removeAllListeners = () =>
+      notImplemented("parentPort.removeAllListeners");
+
+    parentPort.addEventListener("offline", () => {
+      parentPort.emit("close");
+    });
+  }
+};
 
 export function getEnvironmentData(key: unknown) {
-  notImplemented("getEnvironmentData");
   return environmentData.get(key);
 }
 
 export function setEnvironmentData(key: unknown, value?: unknown) {
-  notImplemented("setEnvironmentData");
   if (value === undefined) {
     environmentData.delete(key);
   } else {
@@ -226,7 +248,7 @@ export {
   workerData,
 };
 
-export default {
+const defaultExport = {
   markAsUntransferable,
   moveMessagePortToContext,
   receiveMessageOnPort,
@@ -243,3 +265,5 @@ export default {
   parentPort,
   isMainThread,
 };
+
+export default defaultExport;