diff --git a/packages/rule-tester/src/RuleTester.ts b/packages/rule-tester/src/RuleTester.ts
index c81f7296e189..25f572952719 100644
--- a/packages/rule-tester/src/RuleTester.ts
+++ b/packages/rule-tester/src/RuleTester.ts
@@ -210,7 +210,7 @@ export class RuleTester extends TestFramework {
     // file name (`foo.ts`), don't change the base path.
     if (
       filename != null &&
-      (path.isAbsolute(filename) || filename.startsWith('..'))
+      (path.isAbsolute(filename) || path.normalize(filename).startsWith('..'))
     ) {
       basePath = path.parse(
         path.resolve(basePath ?? process.cwd(), filename),
diff --git a/packages/rule-tester/tests/filename.test.ts b/packages/rule-tester/tests/filename.test.ts
index 0e9656702e95..8f3442cf670a 100644
--- a/packages/rule-tester/tests/filename.test.ts
+++ b/packages/rule-tester/tests/filename.test.ts
@@ -57,6 +57,42 @@ describe('rule tester filename', () => {
         errors: [{ messageId: 'foo' }],
         filename: '../foo.js',
       },
+      {
+        name: 'non-normalized relative path starting with ./',
+        code: '_',
+        errors: [{ messageId: 'foo' }],
+        filename: './../../escaped/cwd/file.ts',
+      },
+      {
+        name: 'non-normalized relative path ./../',
+        code: '_',
+        errors: [{ messageId: 'foo' }],
+        filename: './../foo.js',
+      },
+      {
+        name: 'non-normalized relative path with multiple ./',
+        code: '_',
+        errors: [{ messageId: 'foo' }],
+        filename: '././../foo.js',
+      },
+      {
+        name: 'non-normalized path a/../../',
+        code: '_',
+        errors: [{ messageId: 'foo' }],
+        filename: 'a/../../file.ts',
+      },
+      {
+        name: 'non-normalized path a/b/../c',
+        code: '_',
+        errors: [{ messageId: 'foo' }],
+        filename: 'a/b/../c',
+      },
+      {
+        name: 'non-normalized path with multiple slashes',
+        code: '_',
+        errors: [{ messageId: 'foo' }],
+        filename: 'a/////////////../../../b',
+      },
     ],
     valid: [],
   });
@@ -81,6 +117,42 @@ describe('rule tester filename', () => {
         errors: [{ messageId: 'foo' }],
         filename: '../foo.js',
       },
+      {
+        name: 'non-normalized relative path starting with ./',
+        code: '_',
+        errors: [{ messageId: 'foo' }],
+        filename: './../../escaped/cwd/file.ts',
+      },
+      {
+        name: 'non-normalized relative path ./../',
+        code: '_',
+        errors: [{ messageId: 'foo' }],
+        filename: './../foo.js',
+      },
+      {
+        name: 'non-normalized relative path with multiple ./',
+        code: '_',
+        errors: [{ messageId: 'foo' }],
+        filename: '././../foo.js',
+      },
+      {
+        name: 'non-normalized path a/../../',
+        code: '_',
+        errors: [{ messageId: 'foo' }],
+        filename: 'a/../../file.ts',
+      },
+      {
+        name: 'non-normalized path a/b/../c',
+        code: '_',
+        errors: [{ messageId: 'foo' }],
+        filename: 'a/b/../c',
+      },
+      {
+        name: 'non-normalized path with multiple slashes',
+        code: '_',
+        errors: [{ messageId: 'foo' }],
+        filename: 'a/////////////../../../b',
+      },
     ],
     valid: [],
   });