diff --git a/frontend/src/components/project/ProjectList.test.tsx b/frontend/src/components/project/ProjectList.test.tsx
new file mode 100644
index 00000000000..a416bd98ca3
--- /dev/null
+++ b/frontend/src/components/project/ProjectList.test.tsx
@@ -0,0 +1,87 @@
+/*
+ * Copyright 2025 The Kubernetes Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { describe, expect, it } from 'vitest';
+import App from '../../App';
+import { groupNamespacesIntoProjects } from './ProjectList';
+import { PROJECT_ID_LABEL } from './projectUtils';
+
+// cyclic imports fix
+// eslint-disable-next-line no-unused-vars
+const _dont_delete_me = App;
+
+function ns(name: string, opts: { project?: string; cluster?: string } = {}) {
+  return {
+    metadata: {
+      name,
+      labels: opts.project ? { [PROJECT_ID_LABEL]: opts.project } : undefined,
+    },
+    cluster: opts.cluster ?? 'cluster-a',
+  };
+}
+
+describe('groupNamespacesIntoProjects', () => {
+  it('groups namespaces by project id', () => {
+    const projects = groupNamespacesIntoProjects([
+      ns('app-prod', { project: 'app' }),
+      ns('app-staging', { project: 'app' }),
+      ns('billing', { project: 'billing' }),
+    ]);
+
+    expect(projects).toEqual([
+      { id: 'app', namespaces: ['app-prod', 'app-staging'], clusters: ['cluster-a'] },
+      { id: 'billing', namespaces: ['billing'], clusters: ['cluster-a'] },
+    ]);
+  });
+
+  it('collects clusters from every namespace in a project', () => {
+    const projects = groupNamespacesIntoProjects([
+      ns('shared', { project: 'app', cluster: 'cluster-a' }),
+      ns('shared', { project: 'app', cluster: 'cluster-b' }),
+    ]);
+
+    expect(projects).toHaveLength(1);
+    expect(projects[0].namespaces).toEqual(['shared']);
+    expect(projects[0].clusters).toEqual(['cluster-a', 'cluster-b']);
+  });
+
+  // Regression test for #5254: a namespace without metadata.labels reached
+  // the inner groupBy iteratee through a stale react-query cache and crashed
+  // the Projects page with
+  //   TypeError: Cannot read properties of undefined (reading 'headlamp.dev/project-id')
+  it('skips namespaces with no labels instead of crashing', () => {
+    expect(() =>
+      groupNamespacesIntoProjects([ns('labelled', { project: 'app' }), ns('unlabelled')])
+    ).not.toThrow();
+
+    const projects = groupNamespacesIntoProjects([
+      ns('labelled', { project: 'app' }),
+      ns('unlabelled'),
+    ]);
+    expect(projects).toEqual([{ id: 'app', namespaces: ['labelled'], clusters: ['cluster-a'] }]);
+  });
+
+  it('skips namespaces whose labels object is present but has no project id', () => {
+    const projects = groupNamespacesIntoProjects([
+      {
+        metadata: { name: 'other', labels: { 'app.kubernetes.io/name': 'x' } },
+        cluster: 'cluster-a',
+      },
+      ns('mine', { project: 'app' }),
+    ]);
+    expect(projects).toEqual([{ id: 'app', namespaces: ['mine'], clusters: ['cluster-a'] }]);
+  });
+});
diff --git a/frontend/src/components/project/ProjectList.tsx b/frontend/src/components/project/ProjectList.tsx
index 2ae4c80e15c..22a01d7ac2c 100644
--- a/frontend/src/components/project/ProjectList.tsx
+++ b/frontend/src/components/project/ProjectList.tsx
@@ -29,6 +29,27 @@ import { NewProjectPopup } from './NewProjectPopup';
 import { getHealthIcon, getResourcesHealth, PROJECT_ID_LABEL } from './projectUtils';
 import { useProjectItems } from './useProjectResources';
 
+// The labelSelector on Namespace.useList filters at the API level, but the
+// returned list can still transiently include items without metadata.labels
+// populated (multi-cluster fan-out, react-query cache during a label
+// removal). Without the filter below an unguarded access crashed the
+// Projects page. See issue #5254.
+export function groupNamespacesIntoProjects(
+  namespaces: ReadonlyArray<{
+    metadata: { name: string; labels?: Record<string, string> };
+    cluster: string;
+  }>
+): ProjectDefinition[] {
+  const labelled = namespaces.filter(n => n.metadata.labels?.[PROJECT_ID_LABEL]);
+  return Object.entries(groupBy(labelled, n => n.metadata.labels![PROJECT_ID_LABEL])).map(
+    ([id, ns]) => ({
+      id,
+      namespaces: uniq(ns.map(it => it.metadata.name)),
+      clusters: uniq(ns.map(it => it.cluster)),
+    })
+  );
+}
+
 const useProjects = (): ProjectDefinition[] => {
   const clusterConf = useClustersConf();
   const clusters = Object.values(clusterConf ?? {});
@@ -38,19 +59,7 @@ const useProjects = (): ProjectDefinition[] => {
     labelSelector: PROJECT_ID_LABEL,
   });
 
-  const projects = useMemo(
-    () =>
-      Object.entries(groupBy(namespaces, n => n.metadata.labels![PROJECT_ID_LABEL])).map(
-        ([name, namespaces]) => ({
-          id: name,
-          namespaces: uniq(namespaces.map(it => it.metadata.name)),
-          clusters: uniq(namespaces.map(it => it.cluster)),
-        })
-      ),
-    [namespaces]
-  );
-
-  return projects;
+  return useMemo(() => groupNamespacesIntoProjects(namespaces ?? []), [namespaces]);
 };
 
 export const useProject = (name: string) => {