diff --git a/lib/internal/test_runner/runner.js b/lib/internal/test_runner/runner.js
index f9442be8ed164b..574d496674350b 100644
--- a/lib/internal/test_runner/runner.js
+++ b/lib/internal/test_runner/runner.js
@@ -108,6 +108,11 @@ let debug = require('internal/util/debuglog').debuglog('test_runner', (fn) => {
 });
 
 const kIsolatedProcessName = Symbol('kIsolatedProcessName');
+// NODE_TEST_CONTEXT is set on a child's environment in runTestFile(). It is
+// inherited at process creation, before any user code runs, and is never
+// mutated in-process, so capturing it once keeps run() from reading ambient
+// process state on every call.
+const isTestRunnerChildProcess = process.env.NODE_TEST_CONTEXT !== undefined;
 const kFilterArgs = [
   '--test',
   '--experimental-test-coverage',
@@ -193,7 +198,8 @@ function getRunArgs(path, { forceExit,
                             randomize,
                             randomSeed,
                             root: { timeout },
-                            cwd }) {
+                            cwd,
+                            processExecArgv }) {
   const processNodeOptions = getOptionsAsFlagsFromBinding();
   const runArgs = ArrayPrototypeFilter(processNodeOptions, filterExecArgv);
 
@@ -207,7 +213,7 @@ function getRunArgs(path, { forceExit,
    */
   const nodeOptionsSet = new SafeSet(processNodeOptions);
   const unknownProcessExecArgv = ArrayPrototypeFilter(
-    process.execArgv,
+    processExecArgv,
     (arg) => !nodeOptionsSet.has(arg),
   );
   ArrayPrototypePushApply(runArgs, unknownProcessExecArgv);
@@ -496,7 +502,7 @@ function runTestFile(path, filesWatcher, opts) {
   const subtest = opts.root.createSubtest(FileTest, testPath, testOpts, async (t) => {
     const args = getRunArgs(path, opts);
     const stdio = ['pipe', 'pipe', 'pipe'];
-    const env = { __proto__: null, NODE_TEST_CONTEXT: 'child-v8', ...(opts.env || process.env) };
+    const env = { __proto__: null, NODE_TEST_CONTEXT: 'child-v8', ...(opts.env || opts.processEnv) };
 
     // Acquire a worker ID from the pool for process isolation mode
     let workerId;
@@ -967,6 +973,17 @@ function run(options = kEmptyObject) {
     testFiles.length,
   );
 
+  // Capture references to ambient process state once, at the run() boundary, so
+  // the exec args and environment forwarded to child test processes are fixed
+  // here rather than re-read later inside the per-file subtest callback.
+  // processExecArgv carries the parent's V8/exec-only flags (e.g.
+  // --allow-natives-syntax) and is distinct from the public `execArgv` option;
+  // processEnv is what children inherit when the `env` option is omitted. These
+  // stay live references (not copies) so later process.env mutations remain
+  // visible when each child's env is built.
+  const processExecArgv = process.execArgv;
+  const processEnv = process.env;
+
   const opts = {
     __proto__: null,
     root,
@@ -990,10 +1007,12 @@ function run(options = kEmptyObject) {
     randomize,
     randomSeed,
     testFiles,
+    processExecArgv,
+    processEnv,
   };
 
   if (isolation === 'process') {
-    if (process.env.NODE_TEST_CONTEXT !== undefined) {
+    if (isTestRunnerChildProcess) {
       process.emitWarning('node:test run() is being called recursively within a test file. skipping running files.');
       root.postRun();
       return root.reporter;
diff --git a/test/fixtures/test-runner/process-env-inherited.js b/test/fixtures/test-runner/process-env-inherited.js
new file mode 100644
index 00000000000000..485d4c3b4ab676
--- /dev/null
+++ b/test/fixtures/test-runner/process-env-inherited.js
@@ -0,0 +1,6 @@
+const { test } = require('node:test');
+
+test('process.env is inherited when no env option is provided', (t) => {
+    t.assert.strictEqual(process.env.INHERITED_VAR, 'XYZ', 'parent env var should be inherited by default');
+    t.assert.strictEqual(process.env.NODE_TEST_CONTEXT, 'child-v8', 'NODE_TEST_CONTEXT should be set by run()');
+});
diff --git a/test/parallel/test-runner-run.mjs b/test/parallel/test-runner-run.mjs
index b6eb6b6af51877..43310261fdeb1b 100644
--- a/test/parallel/test-runner-run.mjs
+++ b/test/parallel/test-runner-run.mjs
@@ -836,6 +836,22 @@ describe('env', () => {
     delete process.env.ABC;
   });
 
+  it('should inherit process.env when the env option is omitted', async () => {
+    // Set a variable on the main process env and confirm the spawned test
+    // inherits it when the env option is omitted (the default behavior).
+    process.env.INHERITED_VAR = 'XYZ';
+
+    try {
+      const stream = run({ files: [join(testFixtures, 'process-env-inherited.js')] });
+      stream.on('test:fail', common.mustNotCall());
+      stream.on('test:pass', common.mustCall(1));
+      // eslint-disable-next-line no-unused-vars
+      for await (const _ of stream);
+    } finally {
+      delete process.env.INHERITED_VAR;
+    }
+  });
+
   it('should throw error when env is specified with isolation=none', async () => {
     assert.throws(() => run({ env: { foo: 'bar' }, isolation: 'none' }), {
       code: 'ERR_INVALID_ARG_VALUE',