diff --git a/.changeset/happy-planets-lick.md b/.changeset/happy-planets-lick.md
new file mode 100644
index 00000000000..214226191d9
--- /dev/null
+++ b/.changeset/happy-planets-lick.md
@@ -0,0 +1,5 @@
+---
+"@firebase/rules-unit-testing": patch
+---
+
+Fix assertFails not correctly catching RTDB permission denied errors (#4667).
diff --git a/packages/rules-unit-testing/src/api/index.ts b/packages/rules-unit-testing/src/api/index.ts
index 0ea93425c24..f5138d52505 100644
--- a/packages/rules-unit-testing/src/api/index.ts
+++ b/packages/rules-unit-testing/src/api/index.ts
@@ -583,7 +583,8 @@ export function assertFails(pr: Promise<any>): any {
       const isPermissionDenied =
         errCode === 'permission-denied' ||
         errCode === 'permission_denied' ||
-        errMessage.indexOf('permission_denied') >= 0;
+        errMessage.indexOf('permission_denied') >= 0 || 
+        errMessage.indexOf('permission denied') >= 0;
 
       if (!isPermissionDenied) {
         return Promise.reject(
diff --git a/packages/rules-unit-testing/test/database.test.ts b/packages/rules-unit-testing/test/database.test.ts
index eab354c9cf8..7117f1bc233 100644
--- a/packages/rules-unit-testing/test/database.test.ts
+++ b/packages/rules-unit-testing/test/database.test.ts
@@ -128,6 +128,31 @@ describe('Testing Module Tests', function () {
       .catch(() => {});
   });
 
+  it('assertFails() if message is Permission denied', async function () {
+    const success = Promise.resolve('success');
+    const permissionDenied = Promise.reject({
+      message: 'Permission denied'
+    });
+    const otherFailure = Promise.reject('failure');
+    await firebase
+      .assertFails(success)
+      .then(() => {
+        throw new Error('Expected success to fail.');
+      })
+      .catch(() => {});
+
+    await firebase.assertFails(permissionDenied).catch(() => {
+      throw new Error('Expected permissionDenied to succeed.');
+    });
+
+    await firebase
+      .assertFails(otherFailure)
+      .then(() => {
+        throw new Error('Expected otherFailure to fail.');
+      })
+      .catch(() => {});
+  });
+
   it('discoverEmulators() finds all running emulators', async () => {
     const options = await firebase.discoverEmulators();