diff --git a/doc/api/cli.md b/doc/api/cli.md
index b7ee385555d468..7bbbac94dd75e3 100644
--- a/doc/api/cli.md
+++ b/doc/api/cli.md
@@ -912,6 +912,18 @@ generated as part of the test runner output. If no tests are run, a coverage
 report is not generated. See the documentation on
 [collecting code coverage from tests][] for more details.
 
+### `--experimental-test-isolation`
+
+<!-- YAML
+added:
+  - REPLACEME
+-->
+
+When running tests with the `node:test` module,
+each test file is run in its own process.
+Running `--experimental-test-isolation=none` will run all
+files in the same process.
+
 ### `--experimental-vm-modules`
 
 <!-- YAML
@@ -2505,6 +2517,7 @@ Node.js options that are allowed are:
 * `--experimental-policy`
 * `--experimental-shadow-realm`
 * `--experimental-specifier-resolution`
+* `--experimental-test-isolation`
 * `--experimental-top-level-await`
 * `--experimental-vm-modules`
 * `--experimental-wasi-unstable-preview1`
diff --git a/doc/api/test.md b/doc/api/test.md
index d348061107ce11..05283cec318d5b 100644
--- a/doc/api/test.md
+++ b/doc/api/test.md
@@ -1125,6 +1125,9 @@ changes:
     number. If a nullish value is provided, each process gets its own port,
     incremented from the primary's `process.debugPort`.
     **Default:** `undefined`.
+  * `isolation` {string} If `'process'`, each test file is run in
+    its own process. If `'none'`, all test files run in the same proccess.
+    **Default:** `'process'`.
   * `only`: {boolean} If truthy, the test context will only run tests that
     have the `only` option set
   * `setup` {Function} A function that accepts the `TestsStream` instance
diff --git a/lib/internal/main/test_runner.js b/lib/internal/main/test_runner.js
index 4f3f067cc74242..3dec04ee5c56af 100644
--- a/lib/internal/main/test_runner.js
+++ b/lib/internal/main/test_runner.js
@@ -25,6 +25,8 @@ let debug = require('internal/util/debuglog').debuglog('test_runner', (fn) => {
 prepareMainThreadExecution(false);
 markBootstrapComplete();
 
+
+const isolation = getOptionValue('--experimental-test-isolation');
 let concurrency = getOptionValue('--test-concurrency') || true;
 let inspectPort;
 
@@ -65,6 +67,7 @@ const timeout = getOptionValue('--test-timeout') || Infinity;
 const options = {
   concurrency,
   inspectPort,
+  isolation,
   watch: getOptionValue('--watch'),
   setup: setupTestReporters,
   timeout,
diff --git a/lib/internal/main/tests_entry.js b/lib/internal/main/tests_entry.js
new file mode 100644
index 00000000000000..49e4bace78abb1
--- /dev/null
+++ b/lib/internal/main/tests_entry.js
@@ -0,0 +1,22 @@
+'use strict';
+const {
+  prepareMainThreadExecution,
+  markBootstrapComplete,
+} = require('internal/process/pre_execution');
+const { getOptionValue } = require('internal/options');
+const { getCWDURL } = require('internal/util');
+const { pathToFileURL } = require('internal/url');
+const { initGlobalRoot } = require('internal/test_runner/harness');
+
+prepareMainThreadExecution(false);
+markBootstrapComplete();
+
+const concurrency = getOptionValue('--test-concurrency') || true;
+const parentURL = getCWDURL().href;
+const files = process.env.NODE_TEST_FILES.split(',');
+
+const { esmLoader } = require('internal/process/esm_loader');
+initGlobalRoot({ __proto__: null, concurrency });
+for (const path of files) {
+  esmLoader.import(pathToFileURL(path), parentURL, { __proto__: null });
+}
diff --git a/lib/internal/test_runner/harness.js b/lib/internal/test_runner/harness.js
index 469ca903c7048c..04fcec822b29e6 100644
--- a/lib/internal/test_runner/harness.js
+++ b/lib/internal/test_runner/harness.js
@@ -198,9 +198,9 @@ function setup(root) {
 
 let globalRoot;
 let reportersSetup;
-function getGlobalRoot() {
+function initGlobalRoot(options = kEmptyObject) {
   if (!globalRoot) {
-    globalRoot = createTestTree();
+    globalRoot = createTestTree(options);
     globalRoot.reporter.on('test:fail', (data) => {
       if (data.todo === undefined || data.todo === false) {
         process.exitCode = kGenericUserError;
@@ -214,13 +214,13 @@ function getGlobalRoot() {
 
 async function startSubtest(subtest) {
   await reportersSetup;
-  getGlobalRoot().harness.bootstrapComplete = true;
+  subtest.root.harness.bootstrapComplete = true;
   await subtest.start();
 }
 
 function runInParentContext(Factory) {
   function run(name, options, fn, overrides) {
-    const parent = testResources.get(executionAsyncId()) || getGlobalRoot();
+    const parent = testResources.get(executionAsyncId()) || initGlobalRoot();
     const subtest = parent.createSubtest(Factory, name, options, fn, overrides);
     if (!(parent instanceof Suite)) {
       return startSubtest(subtest);
@@ -252,7 +252,7 @@ function runInParentContext(Factory) {
 
 function hook(hook) {
   return (fn, options) => {
-    const parent = testResources.get(executionAsyncId()) || getGlobalRoot();
+    const parent = testResources.get(executionAsyncId()) || initGlobalRoot();
     parent.createHook(hook, fn, {
       __proto__: null,
       ...options,
@@ -265,6 +265,7 @@ function hook(hook) {
 
 module.exports = {
   createTestTree,
+  initGlobalRoot,
   test: runInParentContext(Test),
   describe: runInParentContext(Suite),
   it: runInParentContext(Test),
diff --git a/lib/internal/test_runner/runner.js b/lib/internal/test_runner/runner.js
index 242e807fa68883..8813ca0d559ee9 100644
--- a/lib/internal/test_runner/runner.js
+++ b/lib/internal/test_runner/runner.js
@@ -50,6 +50,7 @@ const {
   validateBoolean,
   validateFunction,
   validateObject,
+  validateOneOf,
   validateInteger,
 } = require('internal/validators');
 const { getInspectPort, isUsingInspector, isInspectorMessage } = require('internal/util/inspector');
@@ -80,8 +81,9 @@ const {
 } = internalBinding('errors');
 
 const kFilterArgs = ['--test', '--experimental-test-coverage', '--watch'];
-const kFilterArgValues = ['--test-reporter', '--test-reporter-destination'];
+const kFilterArgValues = ['--test-reporter', '--test-reporter-destination', '--experimental-test-isolation'];
 const kDiagnosticsFilterArgs = ['tests', 'suites', 'pass', 'fail', 'cancelled', 'skipped', 'todo', 'duration_ms'];
+const kIsolatedProcessName = '<root>';
 
 const kCanceledTests = new SafeSet()
   .add(kCancelledByParent).add(kAborted).add(kTestTimeoutFailure);
@@ -112,7 +114,7 @@ function filterExecArgv(arg, i, arr) {
   !ArrayPrototypeSome(kFilterArgValues, (p) => arg === p || (i > 0 && arr[i - 1] === p) || StringPrototypeStartsWith(arg, `${p}=`));
 }
 
-function getRunArgs(path, { inspectPort, testNamePatterns, only }) {
+function getRunArgs(path, { inspectPort, testNamePatterns, only, isolation }) {
   const argv = ArrayPrototypeFilter(process.execArgv, filterExecArgv);
   if (isUsingInspector()) {
     ArrayPrototypePush(argv, `--inspect-port=${getInspectPort(inspectPort)}`);
@@ -123,7 +125,9 @@ function getRunArgs(path, { inspectPort, testNamePatterns, only }) {
   if (only === true) {
     ArrayPrototypePush(argv, '--test-only');
   }
-  ArrayPrototypePush(argv, path);
+  if (isolation !== 'none') {
+    ArrayPrototypePush(argv, path);
+  }
 
   return argv;
 }
@@ -144,11 +148,12 @@ class FileTest extends Test {
 
   constructor(options) {
     super(options);
+    const file = this.name === kIsolatedProcessName ? resolve('') : resolve(this.name);
     this.loc ??= {
       __proto__: null,
       line: 1,
       column: 1,
-      file: resolve(this.name),
+      file,
     };
   }
 
@@ -330,6 +335,10 @@ function runTestFile(path, filesWatcher, opts) {
     if (opts.root.harness.shouldColorizeTestFiles) {
       env.FORCE_COLOR = '1';
     }
+    if (opts.isolation === 'none') {
+      args.push('--experimental-test-isolation=none');
+      env.NODE_TEST_FILES = ArrayPrototypeJoin(opts.testFiles, ',');
+    }
 
     const child = spawn(process.execPath, args, { __proto__: null, signal: t.signal, encoding: 'utf8', env, stdio });
     if (watchMode) {
@@ -397,29 +406,40 @@ function runTestFile(path, filesWatcher, opts) {
   return subtest.start();
 }
 
-function watchFiles(testFiles, opts) {
+function watchFiles(opts) {
   const runningProcesses = new SafeMap();
   const runningSubtests = new SafeMap();
   const watcher = new FilesWatcher({ __proto__: null, debounce: 200, mode: 'filter', signal: opts.signal });
   const filesWatcher = { __proto__: null, watcher, runningProcesses, runningSubtests };
 
+  async function stopRunning(file) {
+    const runningProcess = runningProcesses.get(file);
+    if (runningProcess) {
+      runningProcess.kill();
+      await once(runningProcess, 'exit');
+    }
+    if (!runningSubtests.size) {
+      // Reset the topLevel counter
+      opts.root.harness.counters.topLevel = 0;
+    }
+    await runningSubtests.get(file);
+    runningSubtests.set(file, runTestFile(file, filesWatcher, opts));
+  }
+
   watcher.on('changed', ({ owners }) => {
+    if (opts.isolation === 'none') {
+      PromisePrototypeThen(stopRunning(kIsolatedProcessName), undefined, (error) => {
+        triggerUncaughtException(error, true /* fromPromise */);
+      });
+      return;
+    }
     watcher.unfilterFilesOwnedBy(owners);
-    PromisePrototypeThen(SafePromiseAllReturnVoid(testFiles, async (file) => {
+    PromisePrototypeThen(SafePromiseAllReturnVoid(opts.testFiles, async (file) => {
+      console.log(owners, opts.testFiles, opts.isolation);
       if (!owners.has(file)) {
         return;
       }
-      const runningProcess = runningProcesses.get(file);
-      if (runningProcess) {
-        runningProcess.kill();
-        await once(runningProcess, 'exit');
-      }
-      if (!runningSubtests.size) {
-        // Reset the topLevel counter
-        opts.root.harness.counters.topLevel = 0;
-      }
-      await runningSubtests.get(file);
-      runningSubtests.set(file, runTestFile(file, filesWatcher, opts));
+      await stopRunning(file);
     }, undefined, (error) => {
       triggerUncaughtException(error, true /* fromPromise */);
     }));
@@ -439,7 +459,7 @@ function watchFiles(testFiles, opts) {
 function run(options = kEmptyObject) {
   validateObject(options, 'options');
 
-  let { testNamePatterns, shard } = options;
+  let { testNamePatterns, shard, isolation } = options;
   const { concurrency, timeout, signal, files, inspectPort, watch, setup, only } = options;
 
   if (files != null) {
@@ -470,6 +490,10 @@ function run(options = kEmptyObject) {
   if (setup != null) {
     validateFunction(setup, 'options.setup');
   }
+  isolation ||= 'process';
+  if (isolation != null) {
+    validateOneOf(isolation, 'options.isolation', ['process', 'none']);
+  }
   if (testNamePatterns != null) {
     if (!ArrayIsArray(testNamePatterns)) {
       testNamePatterns = [testNamePatterns];
@@ -501,13 +525,18 @@ function run(options = kEmptyObject) {
 
   let postRun = () => root.postRun();
   let filesWatcher;
-  const opts = { __proto__: null, root, signal, inspectPort, testNamePatterns, only };
+  const opts = { __proto__: null, root, signal, inspectPort, testNamePatterns, only, isolation, testFiles };
   if (watch) {
-    filesWatcher = watchFiles(testFiles, opts);
+    filesWatcher = watchFiles(opts);
     postRun = undefined;
   }
   const runFiles = () => {
     root.harness.bootstrapComplete = true;
+    if (isolation === 'none') {
+      const subtest = runTestFile(kIsolatedProcessName, filesWatcher, opts);
+      filesWatcher?.runningSubtests.set(kIsolatedProcessName, subtest);
+      return subtest;
+    }
     return SafePromiseAllSettledReturnVoid(testFiles, (path) => {
       const subtest = runTestFile(path, filesWatcher, opts);
       filesWatcher?.runningSubtests.set(path, subtest);
diff --git a/lib/internal/test_runner/test.js b/lib/internal/test_runner/test.js
index ee2c345e7a0403..bea4b2c1c29b63 100644
--- a/lib/internal/test_runner/test.js
+++ b/lib/internal/test_runner/test.js
@@ -70,7 +70,6 @@ const kHookFailure = 'hookFailed';
 const kDefaultTimeout = null;
 const noop = FunctionPrototype;
 const kShouldAbort = Symbol('kShouldAbort');
-const kFilename = process.argv?.[1];
 const kHookNames = ObjectSeal(['before', 'after', 'beforeEach', 'afterEach']);
 const kUnwrapErrors = new SafeSet()
   .add(kTestCodeFailure).add(kHookFailure)
@@ -349,7 +348,7 @@ class Test extends AsyncResource {
       this.diagnostic(warning);
     }
 
-    if (loc === undefined || kFilename === undefined) {
+    if (loc === undefined) {
       this.loc = undefined;
     } else {
       this.loc = {
@@ -826,11 +825,13 @@ class Test extends AsyncResource {
       details.type = this.reportedType;
     }
 
+    const isTopLevel = this.nesting === 0;
+    const testNumber = isTopLevel ? this.root.harness.counters.topLevel : this.testNumber;
     if (this.passed) {
-      this.reporter.ok(this.nesting, this.loc, this.testNumber, this.name, details, directive);
+      this.reporter.ok(this.nesting, this.loc, testNumber, this.name, details, directive);
     } else {
       details.error = this.error;
-      this.reporter.fail(this.nesting, this.loc, this.testNumber, this.name, details, directive);
+      this.reporter.fail(this.nesting, this.loc, testNumber, this.name, details, directive);
     }
 
     for (let i = 0; i < this.diagnostics.length; i++) {
diff --git a/lib/internal/test_runner/utils.js b/lib/internal/test_runner/utils.js
index 184d44dce6c162..955ea333f8e3e3 100644
--- a/lib/internal/test_runner/utils.js
+++ b/lib/internal/test_runner/utils.js
@@ -24,8 +24,7 @@ const {
 const { AsyncResource } = require('async_hooks');
 const { relative } = require('path');
 const { createWriteStream } = require('fs');
-const { pathToFileURL } = require('internal/url');
-const { createDeferredPromise } = require('internal/util');
+const { createDeferredPromise, getCWDURL } = require('internal/util');
 const { getOptionValue } = require('internal/options');
 const { green, yellow, red, white, shouldColorize } = require('internal/util/colors');
 
@@ -146,14 +145,7 @@ async function getReportersMap(reporters, destinations) {
     let reporter = tryBuiltinReporter(name);
 
     if (reporter === undefined) {
-      let parentURL;
-
-      try {
-        parentURL = pathToFileURL(process.cwd() + '/').href;
-      } catch {
-        parentURL = 'file:///';
-      }
-
+      const parentURL = getCWDURL().href;
       const { esmLoader } = require('internal/process/esm_loader');
       reporter = await esmLoader.import(name, parentURL, { __proto__: null });
     }
diff --git a/src/node.cc b/src/node.cc
index b75296dd85e0ae..909a55749eafda 100644
--- a/src/node.cc
+++ b/src/node.cc
@@ -401,6 +401,11 @@ MaybeLocal<Value> StartExecution(Environment* env, StartExecutionCallback cb) {
     return StartExecution(env, "internal/main/check_syntax");
   }
 
+  if (env->options()->test_isolation == "none" &&
+      env->env_vars()->Get("NODE_TEST_FILES").IsJust()) {
+    return StartExecution(env, "internal/main/tests_entry");
+  }
+
   if (env->options()->test_runner) {
     return StartExecution(env, "internal/main/test_runner");
   }
diff --git a/src/node_options.cc b/src/node_options.cc
index 7b5152172c5ce7..379b893a7c85ad 100644
--- a/src/node_options.cc
+++ b/src/node_options.cc
@@ -641,6 +641,10 @@ EnvironmentOptionsParser::EnvironmentOptionsParser() {
             "run test at specific shard",
             &EnvironmentOptions::test_shard,
             kAllowedInEnvvar);
+  AddOption("--experimental-test-isolation",
+            "isolation mode of test runner",
+            &EnvironmentOptions::test_isolation,
+            kAllowedInEnvvar);
   AddOption("--test-udp-no-try-send", "",  // For testing only.
             &EnvironmentOptions::test_udp_no_try_send);
   AddOption("--throw-deprecation",
diff --git a/src/node_options.h b/src/node_options.h
index a0b56ebacb436c..5f4d1198aa6b95 100644
--- a/src/node_options.h
+++ b/src/node_options.h
@@ -172,6 +172,7 @@ class EnvironmentOptions : public Options {
   bool test_only = false;
   bool test_udp_no_try_send = false;
   std::string test_shard;
+  std::string test_isolation;
   bool throw_deprecation = false;
   bool trace_atomics_wait = false;
   bool trace_deprecation = false;
diff --git a/test/fixtures/test-runner/no-isolation/a.test.js b/test/fixtures/test-runner/no-isolation/a.test.js
new file mode 100644
index 00000000000000..a59c9d4d7786b6
--- /dev/null
+++ b/test/fixtures/test-runner/no-isolation/a.test.js
@@ -0,0 +1,4 @@
+'use strict';
+const test = require('node:test');
+
+test('a', (t, cb) => { globalThis.aCb = cb; });
diff --git a/test/fixtures/test-runner/no-isolation/b.test.js b/test/fixtures/test-runner/no-isolation/b.test.js
new file mode 100644
index 00000000000000..269cbf5725bd54
--- /dev/null
+++ b/test/fixtures/test-runner/no-isolation/b.test.js
@@ -0,0 +1,7 @@
+'use strict';
+const test = require('node:test');
+
+test('b', async () => {
+  globalThis.aCb();
+  delete globalThis.aCb;
+});
diff --git a/test/parallel/test-runner-cli.js b/test/parallel/test-runner-cli.js
index ab6078a4a05d74..f667322db2f571 100644
--- a/test/parallel/test-runner-cli.js
+++ b/test/parallel/test-runner-cli.js
@@ -36,6 +36,20 @@ const testFixtures = fixtures.path('test-runner');
   assert.match(stdout, /ok 6 - this should be executed/);
 }
 
+
+{
+  // Test test files with --experimental-test-isolation=none
+  const args = ['--test', '--experimental-test-isolation=none'];
+  const child = spawnSync(process.execPath, args, { cwd: join(testFixtures, 'no-isolation') });
+
+  assert.strictEqual(child.status, 0);
+  assert.strictEqual(child.signal, null);
+  assert.strictEqual(child.stderr.toString(), '');
+  const stdout = child.stdout.toString();
+  assert.match(stdout, /ok 1 - a/);
+  assert.match(stdout, /ok 2 - b/);
+}
+
 {
   // Same but with a prototype mutation in require scripts.
   const args = ['--require', join(testFixtures, 'protoMutation.js'), '--test'];
diff --git a/test/parallel/test-runner-concurrency.js b/test/parallel/test-runner-concurrency.js
index 2cb01bdc268f19..4399f3182d452b 100644
--- a/test/parallel/test-runner-concurrency.js
+++ b/test/parallel/test-runner-concurrency.js
@@ -96,3 +96,15 @@ test('--test multiple files', { skip: os.availableParallelism() < 3 }, async ()
   assert.strictEqual(stderr, '');
   assert.strictEqual(code, 0);
 });
+
+test('--test multiple files with --test-isolation=none', { skip: os.availableParallelism() < 3 }, async () => {
+  await fs.writeFile(tmpdir.resolve('test-runner-concurrency'), '');
+  const { code, stderr } = await common.spawnPromisified(process.execPath, [
+    '--test',
+    '--experimental-test-isolation=none',
+    fixtures.path('test-runner', 'concurrency', 'a.mjs'),
+    fixtures.path('test-runner', 'concurrency', 'b.mjs'),
+  ]);
+  assert.strictEqual(stderr, '');
+  assert.strictEqual(code, 0);
+});
diff --git a/test/parallel/test-runner-run.mjs b/test/parallel/test-runner-run.mjs
index 8f95de4db16296..e509ff3bf1834d 100644
--- a/test/parallel/test-runner-run.mjs
+++ b/test/parallel/test-runner-run.mjs
@@ -273,6 +273,20 @@ describe('require(\'node:test\').run', { concurrency: true }, () => {
     });
   });
 
+  it('should run multiple files in same process', async () => {
+    const stream = run({
+      files: [
+        join(testFixtures, 'no-isolation/a.test.js'),
+        join(testFixtures, 'no-isolation/b.test.js')],
+      concurrency: 2,
+      isolation: 'none'
+    });
+    stream.on('test:fail', common.mustNotCall());
+    stream.on('test:pass', common.mustCall(2));
+    // eslint-disable-next-line no-unused-vars
+    for await (const _ of stream);
+  });
+
   describe('sharding', () => {
     const shardsTestsFixtures = fixtures.path('test-runner', 'shards');
     const shardsTestsFiles = [