diff --git a/packages/kbn-test/src/functional_test_runner/lib/mocha/decorate_mocha_ui.js b/packages/kbn-test/src/functional_test_runner/lib/mocha/decorate_mocha_ui.js
index 9a1451c6471ef3..1cac852a7e7130 100644
--- a/packages/kbn-test/src/functional_test_runner/lib/mocha/decorate_mocha_ui.js
+++ b/packages/kbn-test/src/functional_test_runner/lib/mocha/decorate_mocha_ui.js
@@ -16,7 +16,7 @@
  * specific language governing permissions and limitations
  * under the License.
  */
-import { sep } from 'path';
+import { relative } from 'path';
 import { REPO_ROOT } from '@kbn/dev-utils';
 import { createAssignmentProxy } from './assignment_proxy';
 import { wrapFunction } from './wrap_function';
@@ -66,7 +66,7 @@ export function decorateMochaUi(lifecycle, context) {
             this._tags = [].concat(this._tags || [], tags);
           };
 
-          const relativeFilePath = this.file.replace(REPO_ROOT + sep, '');
+          const relativeFilePath = relative(REPO_ROOT, this.file);
           this.tags(relativeFilePath);
           this.suiteTag = relativeFilePath; // The tag that uniquely targets this suite/file