diff --git a/lib/internal/errors.js b/lib/internal/errors.js
index 23a54754e2a321..d9550c151c9595 100644
--- a/lib/internal/errors.js
+++ b/lib/internal/errors.js
@@ -1133,6 +1133,11 @@ E('ERR_ACCESS_DENIED',
     return msg;
   },
   Error);
+
+  E('ERR_AMBIGUOUS_MODULE_SYNTAX',
+  'The module syntax is ambiguous. This file mixes CommonJS (require, module.exports) and ES Module features (import, export, or top-level await). Use either CommonJS or ES Modules consistently in the same file.',
+  Error);
+
 E('ERR_AMBIGUOUS_ARGUMENT', 'The "%s" argument is ambiguous. %s', TypeError);
 E('ERR_ARG_NOT_ITERABLE', '%s must be iterable', TypeError);
 E('ERR_ASSERTION', '%s', Error);
diff --git a/test/es-module/test-ambiguous-module-syntax.js b/test/es-module/test-ambiguous-module-syntax.js
new file mode 100644
index 00000000000000..060ddb82543faa
--- /dev/null
+++ b/test/es-module/test-ambiguous-module-syntax.js
@@ -0,0 +1,31 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const { spawnSync } = require('child_process');
+const fixtures = require('../common/fixtures');
+
+// Skip if ESM is not supported in this environment
+common.skipIfESMUnsupported(__filename);
+
+// Fixture that mixes CJS + ESM
+const entry = fixtures.path('es-modules', 'ambiguous-mixed-syntax.js');
+
+const result = spawnSync(process.execPath, [entry], {
+  encoding: 'utf8'
+});
+
+// Node should exit with an error
+assert.notStrictEqual(result.status, 0);
+
+// Error code must be present
+assert.ok(
+  /ERR_AMBIGUOUS_MODULE_SYNTAX/.test(result.stderr),
+  `stderr did not contain ERR_AMBIGUOUS_MODULE_SYNTAX:\n${result.stderr}`
+);
+
+// Your new explanatory message should appear
+assert.ok(
+  /mixes CommonJS/.test(result.stderr),
+  `stderr did not contain expected message:\n${result.stderr}`
+);
diff --git a/test/fixtures/es-modules/ambiguous-mixed-syntax.js b/test/fixtures/es-modules/ambiguous-mixed-syntax.js
new file mode 100644
index 00000000000000..18631e97461564
--- /dev/null
+++ b/test/fixtures/es-modules/ambiguous-mixed-syntax.js
@@ -0,0 +1,8 @@
+// This file intentionally mixes CommonJS and ES Module syntax
+// to trigger ERR_AMBIGUOUS_MODULE_SYNTAX.
+
+// CommonJS
+const fs = require('fs');
+
+// ES Modules
+export const value = 123;