diff --git a/lib/child_process.js b/lib/child_process.js
index 6b411f43f07..3db7ec752be 100644
--- a/lib/child_process.js
+++ b/lib/child_process.js
@@ -741,8 +741,11 @@ function normalizeSpawnArguments(file, args, options) {
     // instead of copying process.env (one interceptor round trip and one
     // getenv() scan per variable). Everything copyProcessEnvToEnv() would
     // propagate below is part of it by definition, and entries of the real
-    // environment block cannot contain null bytes.
-    envPairs = getEnvPairs();
+    // environment block cannot contain null bytes. Undefined if process.env
+    // was reassigned.
+    envPairs = getEnvPairs(process.env);
+  }
+  if (envPairs !== undefined) {
     if (process.platform === 'win32') {
       envPairs = dedupeWindowsEnvPairs(envPairs);
     }
diff --git a/src/process_wrap.cc b/src/process_wrap.cc
index 8ca42a784de..34e10b4b2a7 100644
--- a/src/process_wrap.cc
+++ b/src/process_wrap.cc
@@ -346,9 +346,17 @@ class ProcessWrap : public HandleWrap {
 
   // The current environment as ["KEY=value", ...], i.e. what a spawned
   // child inherits by default, produced in one pass over the environment
-  // block instead of one interceptor round trip per variable.
+  // block instead of one interceptor round trip per variable. Returns
+  // undefined if args[0] is not the env proxy, e.g. because process.env was
+  // reassigned, in which case the caller has to copy that object instead.
   static void GetEnvPairs(const FunctionCallbackInfo<Value>& args) {
     Environment* env = Environment::GetCurrent(args);
+    Local<FunctionTemplate> env_proxy_ctor_template =
+        env->env_proxy_ctor_template();
+    if (env_proxy_ctor_template.IsEmpty() ||
+        !env_proxy_ctor_template->HasInstance(args[0])) {
+      return;
+    }
     Local<Array> pairs;
     if (env->env_vars()->Pairs(env->isolate()).ToLocal(&pairs)) {
       args.GetReturnValue().Set(pairs);
diff --git a/test/parallel/test-child-process-env-reassigned.js b/test/parallel/test-child-process-env-reassigned.js
new file mode 100644
index 00000000000..44cd42fc4c9
--- /dev/null
+++ b/test/parallel/test-child-process-env-reassigned.js
@@ -0,0 +1,103 @@
+'use strict';
+// When no `env` option is given, a child process inherits process.env, even
+// if process.env has been replaced with another object.
+// Refs: https://github.com/nodejs/node/issues/66882
+const common = require('../common');
+const assert = require('assert');
+const {
+  exec,
+  execFile,
+  execFileSync,
+  execSync,
+  fork,
+  spawn,
+  spawnSync,
+} = require('child_process');
+
+if (process.argv[2] === 'child') {
+  process.send(process.env.TEST_REASSIGNED);
+  return;
+}
+
+const printEnv = ['-p', 'process.env.TEST_REASSIGNED'];
+const shellEcho = common.isWindows ?
+  'echo %TEST_REASSIGNED%' :
+  'echo "$TEST_REASSIGNED"';
+
+function runSync(options) {
+  const child = spawnSync(process.execPath, printEnv,
+                          { encoding: 'utf8', ...options });
+  assert.strictEqual(child.status, 0, child.stderr || String(child.error));
+  return child.stdout.trim();
+}
+
+const originalEnv = process.env;
+// Lives in the real environment, but not in the replacement object.
+process.env.TEST_ONLY_IN_ORIGINAL = 'original';
+process.env = { ...process.env, TEST_REASSIGNED: 'reassigned' };
+delete process.env.TEST_ONLY_IN_ORIGINAL;
+
+assert.strictEqual(runSync(), 'reassigned');
+assert.strictEqual(
+  execFileSync(process.execPath, printEnv, { encoding: 'utf8' }).trim(),
+  'reassigned');
+assert.strictEqual(execSync(shellEcho, { encoding: 'utf8' }).trim(),
+                   'reassigned');
+
+{
+  const child = spawnSync(
+    process.execPath,
+    ['-p', 'process.env.TEST_ONLY_IN_ORIGINAL'],
+    { encoding: 'utf8' });
+  assert.strictEqual(child.stdout.trim(), 'undefined');
+}
+
+// The default env matches an explicit { env: process.env }.
+{
+  const printAll = ['-e', 'process.stdout.write(JSON.stringify(process.env))'];
+  const run = (options) => JSON.parse(
+    spawnSync(process.execPath, printAll, { encoding: 'utf8', ...options })
+      .stdout);
+  assert.deepStrictEqual(run(), run({ env: process.env }));
+}
+
+// The replacement object is used for command lookup.
+if (!common.isWindows) {
+  const saved = process.env;
+  process.env = { ...saved, PATH: '/nonexistent' };
+  const child = spawnSync('ls');
+  process.env = saved;
+  assert.strictEqual(child.error?.code, 'ENOENT');
+}
+
+{
+  const child = spawn(process.execPath, printEnv);
+  let out = '';
+  child.stdout.setEncoding('utf8').on('data', (d) => { out += d; });
+  let err = '';
+  child.stderr.setEncoding('utf8').on('data', (d) => { err += d; });
+  child.on('close', common.mustCall((code) => {
+    assert.strictEqual(code, 0, err);
+    assert.strictEqual(out.trim(), 'reassigned');
+  }));
+}
+
+exec(shellEcho, common.mustSucceed((stdout) => {
+  assert.strictEqual(stdout.trim(), 'reassigned');
+}));
+
+execFile(process.execPath, printEnv, common.mustSucceed((stdout) => {
+  assert.strictEqual(stdout.trim(), 'reassigned');
+}));
+
+fork(__filename, ['child']).on('message', common.mustCall((value) => {
+  assert.strictEqual(value, 'reassigned');
+}));
+
+// Putting the original object back restores inheriting the real environment.
+process.env = originalEnv;
+assert.strictEqual(runSync(), 'undefined');
+assert.strictEqual(
+  spawnSync(process.execPath, ['-p', 'process.env.TEST_ONLY_IN_ORIGINAL'],
+            { encoding: 'utf8' }).stdout.trim(),
+  'original');