diff --git a/.changeset/curly-pillows-provide.md b/.changeset/curly-pillows-provide.md
new file mode 100644
index 0000000000..e6a218233b
--- /dev/null
+++ b/.changeset/curly-pillows-provide.md
@@ -0,0 +1,5 @@
+---
+'@backstage/test-utils': patch
+---
+
+JSON serialize and freeze values stored by the `MockStorageApi`.
diff --git a/packages/test-utils/src/testUtils/apis/StorageApi/MockStorageApi.test.ts b/packages/test-utils/src/testUtils/apis/StorageApi/MockStorageApi.test.ts
index a8187cd849..c974198be1 100644
--- a/packages/test-utils/src/testUtils/apis/StorageApi/MockStorageApi.test.ts
+++ b/packages/test-utils/src/testUtils/apis/StorageApi/MockStorageApi.test.ts
@@ -216,4 +216,73 @@ describe('WebStorage Storage API', () => {
       newValue: undefined,
     });
   });
+
+  it('should freeze the snapshot value', async () => {
+    const storage = createMockStorage();
+
+    const data = { foo: 'bar', baz: [{ foo: 'bar' }] };
+    storage.set('foo', data);
+
+    const snapshot = storage.snapshot<typeof data>('foo');
+    expect(snapshot.value).not.toBe(data);
+
+    if (snapshot.presence !== 'present') {
+      throw new Error('Invalid presence');
+    }
+
+    expect(() => {
+      snapshot.value.foo = 'buzz';
+    }).toThrow(/Cannot assign to read only property/);
+    expect(() => {
+      snapshot.value.baz[0].foo = 'buzz';
+    }).toThrow(/Cannot assign to read only property/);
+    expect(() => {
+      snapshot.value.baz.push({ foo: 'buzz' });
+    }).toThrow(/Cannot add property 1, object is not extensible/);
+  });
+
+  it('should freeze observed values', async () => {
+    const storage = createMockStorage();
+
+    const snapshotPromise = new Promise<any>(resolve => {
+      storage.observe$('test').subscribe({
+        next: resolve,
+      });
+    });
+
+    storage.set('test', {
+      foo: {
+        bar: 'baz',
+      },
+    });
+
+    const snapshot = await snapshotPromise;
+    expect(snapshot.presence).toBe('present');
+    expect(() => {
+      snapshot.value!.foo.bar = 'qux';
+    }).toThrow(/Cannot assign to read only property 'bar' of object/);
+  });
+
+  it('should JSON serialize stored values', async () => {
+    const storage = createMockStorage();
+
+    storage.set<any>('test', {
+      foo: {
+        toJSON() {
+          return {
+            bar: 'baz',
+          };
+        },
+      },
+    });
+
+    expect(storage.snapshot('test')).toMatchObject({
+      presence: 'present',
+      value: {
+        foo: {
+          bar: 'baz',
+        },
+      },
+    });
+  });
 });
diff --git a/packages/test-utils/src/testUtils/apis/StorageApi/MockStorageApi.ts b/packages/test-utils/src/testUtils/apis/StorageApi/MockStorageApi.ts
index f71d2602b5..25024f2211 100644
--- a/packages/test-utils/src/testUtils/apis/StorageApi/MockStorageApi.ts
+++ b/packages/test-utils/src/testUtils/apis/StorageApi/MockStorageApi.ts
@@ -84,12 +84,18 @@ export class MockStorageApi implements StorageApi {
   }
 
   async set<T>(key: string, data: T): Promise<void> {
-    this.data[this.getKeyName(key)] = data;
+    const serialized = JSON.parse(JSON.stringify(data), (_key, value) => {
+      if (typeof value === 'object' && value !== null) {
+        Object.freeze(value);
+      }
+      return value;
+    });
+    this.data[this.getKeyName(key)] = serialized;
     this.notifyChanges({
       key,
       presence: 'present',
-      value: data,
-      newValue: data,
+      value: serialized,
+      newValue: serialized,
     });
   }