diff --git a/tools/hermes-parser/js/babel-plugin-syntax-hermes-parser/__tests__/babel-plugin-syntax-hermes-parser-test.js b/tools/hermes-parser/js/babel-plugin-syntax-hermes-parser/__tests__/babel-plugin-syntax-hermes-parser-test.js
index a03108c2b20..3bcf530c55c 100644
--- a/tools/hermes-parser/js/babel-plugin-syntax-hermes-parser/__tests__/babel-plugin-syntax-hermes-parser-test.js
+++ b/tools/hermes-parser/js/babel-plugin-syntax-hermes-parser/__tests__/babel-plugin-syntax-hermes-parser-test.js
@@ -16,7 +16,7 @@ import {transformSync} from '@babel/core';
 import hermesParserPlugin from '../src';
 import * as HermesParser from 'hermes-parser';
 
-const MODULE_PREAMBLE = '"use strict";\n\n';
+const MODULE_PREAMBLE = '// @flow\n\n"use strict";\n\n';
 
 describe('babel-plugin-syntax-hermes-parser', () => {
   test('test basic parsing', () => {
diff --git a/tools/hermes-parser/js/babel-plugin-syntax-hermes-parser/src/index.js b/tools/hermes-parser/js/babel-plugin-syntax-hermes-parser/src/index.js
index a04237fb031..ac4730cfd13 100644
--- a/tools/hermes-parser/js/babel-plugin-syntax-hermes-parser/src/index.js
+++ b/tools/hermes-parser/js/babel-plugin-syntax-hermes-parser/src/index.js
@@ -37,8 +37,9 @@ export default function BabelPluginSyntaxHermesParser(
     parserOverride(code: string) {
       const filename = curFilename;
       if (
-        filename != null &&
-        (filename.endsWith('.ts') || filename.endsWith('.tsx'))
+        !/@flow/.test(code) ||
+        (filename != null &&
+          (filename.endsWith('.ts') || filename.endsWith('.tsx')))
       ) {
         return;
       }