diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2e4ef3c69..cbb136545 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,9 @@
 # Changelog
 
+## 4.0.15
+
+- [556](https://github.com/bvaughn/react-resizable-panels/pull/556): Ignore `defaultLayout` when keys don't match Panel ids
+
 ## 4.0.14
 
 - [555](https://github.com/bvaughn/react-resizable-panels/pull/555): Allow resizable panels to be rendered into a different Window (e.g. popup or frame) by accessing globals through `element.ownerDocument.defaultView`
diff --git a/lib/components/group/Group.test.tsx b/lib/components/group/Group.test.tsx
index 33c625ba6..c72d97a26 100644
--- a/lib/components/group/Group.test.tsx
+++ b/lib/components/group/Group.test.tsx
@@ -3,7 +3,10 @@ import { beforeEach, describe, expect, test, vi } from "vitest";
 import { eventEmitter } from "../../global/mutableState";
 import { moveSeparator } from "../../global/test/moveSeparator";
 import { assert } from "../../utils/assert";
-import { setElementBoundsFunction } from "../../utils/test/mockBoundingClientRect";
+import {
+  setDefaultElementBounds,
+  setElementBoundsFunction
+} from "../../utils/test/mockBoundingClientRect";
 import { Panel } from "../panel/Panel";
 import { Separator } from "../separator/Separator";
 import { Group } from "./Group";
@@ -81,6 +84,60 @@ describe("Group", () => {
     );
   });
 
+  describe("defaultLayout", () => {
+    test("should be ignored if it does not match Panel ids", () => {
+      const onLayoutChange = vi.fn();
+
+      setDefaultElementBounds(new DOMRect(0, 0, 100, 50));
+
+      render(
+        <Group
+          defaultLayout={{
+            foo: 40,
+            bar: 60
+          }}
+          onLayoutChange={onLayoutChange}
+        >
+          <Panel id="bar" />
+          <Panel id="baz" />
+        </Group>
+      );
+
+      expect(onLayoutChange).toHaveBeenCalledTimes(1);
+      expect(onLayoutChange).toHaveBeenCalledWith({
+        bar: 50,
+        baz: 50
+      });
+    });
+
+    test("should be ignored if it does not match Panel ids (mounted within hidden subtree)", () => {
+      const onLayoutChange = vi.fn();
+
+      render(
+        <Group
+          defaultLayout={{
+            foo: 40,
+            bar: 60
+          }}
+          onLayoutChange={onLayoutChange}
+        >
+          <Panel id="bar" />
+          <Panel id="baz" />
+        </Group>
+      );
+
+      expect(onLayoutChange).not.toHaveBeenCalled();
+
+      setDefaultElementBounds(new DOMRect(0, 0, 100, 50));
+
+      expect(onLayoutChange).toHaveBeenCalledTimes(1);
+      expect(onLayoutChange).toHaveBeenCalledWith({
+        bar: 50,
+        baz: 50
+      });
+    });
+  });
+
   describe("groupRef", () => {
     test("should work with an empty Group", () => {
       const onLayoutChange = vi.fn();
diff --git a/lib/components/group/useDefaultLayout.test.tsx b/lib/components/group/useDefaultLayout.test.tsx
index 70df9239b..f05318b57 100644
--- a/lib/components/group/useDefaultLayout.test.tsx
+++ b/lib/components/group/useDefaultLayout.test.tsx
@@ -1,11 +1,11 @@
 import { render, renderHook } from "@testing-library/react";
 import { createRef } from "react";
 import { describe, expect, test, vi } from "vitest";
+import { setDefaultElementBounds } from "../../utils/test/mockBoundingClientRect";
 import { Panel } from "../panel/Panel";
 import { Group } from "./Group";
-import type { GroupImperativeHandle, LayoutStorage } from "./types";
+import { type GroupImperativeHandle, type LayoutStorage } from "./types";
 import { useDefaultLayout } from "./useDefaultLayout";
-import { setDefaultElementBounds } from "../../utils/test/mockBoundingClientRect";
 
 describe("useDefaultLayout", () => {
   test("should read/write from the provided Storage API", () => {
@@ -83,7 +83,7 @@ describe("useDefaultLayout", () => {
   });
 
   // See github.com/bvaughn/react-resizable-panels/pull/540
-  test("should not break when coupled with dynamic layouts", () => {
+  test("should ignore invalid layouts (num panels mismatch)", () => {
     const groupRef = createRef<GroupImperativeHandle>();
 
     setDefaultElementBounds(new DOMRect(0, 0, 100, 50));
@@ -134,4 +134,30 @@ describe("useDefaultLayout", () => {
       }
     `);
   });
+
+  test("should ignore invalid layouts (panel ids mismatch)", () => {
+    setDefaultElementBounds(new DOMRect(0, 0, 100, 50));
+
+    const groupRef = createRef<GroupImperativeHandle>();
+
+    render(
+      <Group
+        defaultLayout={{
+          foo: 40,
+          bar: 60
+        }}
+        groupRef={groupRef}
+      >
+        <Panel id="bar" defaultSize="30%" />
+        <Panel id="baz" />
+      </Group>
+    );
+
+    expect(groupRef.current?.getLayout()).toMatchInlineSnapshot(`
+      {
+        "bar": 30,
+        "baz": 70,
+      }
+    `);
+  });
 });
diff --git a/lib/global/mountGroup.ts b/lib/global/mountGroup.ts
index 674a88d1e..02ab63a97 100644
--- a/lib/global/mountGroup.ts
+++ b/lib/global/mountGroup.ts
@@ -13,6 +13,7 @@ import { calculateDefaultLayout } from "./utils/calculateDefaultLayout";
 import { layoutsEqual } from "./utils/layoutsEqual";
 import { notifyPanelOnResize } from "./utils/notifyPanelOnResize";
 import { objectsEqual } from "./utils/objectsEqual";
+import { validateLayoutKeys } from "./utils/validateLayoutKeys";
 import { validatePanelGroupLayout } from "./utils/validatePanelGroupLayout";
 
 const ownerDocumentReferenceCounts = new Map<Document, number>();
@@ -114,7 +115,7 @@ export function mountGroup(group: RegisteredGroup) {
   // In this case the best we can do is ignore the incoming layout
   let defaultLayout: Layout | undefined = group.defaultLayout;
   if (defaultLayout) {
-    if (group.panels.length !== Object.keys(defaultLayout).length) {
+    if (!validateLayoutKeys(group.panels, defaultLayout)) {
       defaultLayout = undefined;
     }
   }
diff --git a/lib/global/utils/validateLayoutKeys.ts b/lib/global/utils/validateLayoutKeys.ts
new file mode 100644
index 000000000..88af2fd7b
--- /dev/null
+++ b/lib/global/utils/validateLayoutKeys.ts
@@ -0,0 +1,19 @@
+import type { Layout } from "../../components/group/types";
+import type { RegisteredPanel } from "../../components/panel/types";
+
+export function validateLayoutKeys(panels: RegisteredPanel[], layout: Layout) {
+  const panelIds = panels.map((panel) => panel.id);
+  const layoutKeys = Object.keys(layout);
+
+  if (panelIds.length !== layoutKeys.length) {
+    return false;
+  }
+
+  for (const panelId of panelIds) {
+    if (!layoutKeys.includes(panelId)) {
+      return false;
+    }
+  }
+
+  return true;
+}