diff --git a/packages/utils/src/eslint-utils/rule-tester/RuleTester.ts b/packages/utils/src/eslint-utils/rule-tester/RuleTester.ts
index 38a412448567..7d21d51d92fc 100644
--- a/packages/utils/src/eslint-utils/rule-tester/RuleTester.ts
+++ b/packages/utils/src/eslint-utils/rule-tester/RuleTester.ts
@@ -4,6 +4,7 @@ import * as path from 'path';
 import * as semver from 'semver';
 
 import * as TSESLint from '../../ts-eslint';
+import { deepMerge } from '../deepMerge';
 import type { DependencyConstraint } from './dependencyConstraints';
 import { satisfiesAllDependencyConstraints } from './dependencyConstraints';
 
@@ -84,18 +85,19 @@ class RuleTester extends TSESLint.RuleTester {
     });
   }
   private getFilename(testOptions?: TSESLint.ParserOptions): string {
-    const filename = `file.ts${testOptions?.ecmaFeatures?.jsx ? 'x' : ''}`;
-    if (testOptions?.project) {
+    const resolvedOptions = deepMerge(
+      this.#baseOptions.parserOptions,
+      testOptions,
+    ) as TSESLint.ParserOptions;
+    const filename = `file.ts${resolvedOptions.ecmaFeatures?.jsx ? 'x' : ''}`;
+    if (resolvedOptions.project) {
       return path.join(
-        testOptions.tsconfigRootDir != null
-          ? testOptions.tsconfigRootDir
+        resolvedOptions.tsconfigRootDir != null
+          ? resolvedOptions.tsconfigRootDir
           : process.cwd(),
         filename,
       );
-    } else if (this.#baseOptions.parserOptions?.project) {
-      return this.getFilename(this.#baseOptions.parserOptions);
     }
-
     return filename;
   }