diff --git a/.changeset/signed-in-signed-out-protect-error.md b/.changeset/signed-in-signed-out-protect-error.md
new file mode 100644
index 00000000000..e5047145bb4
--- /dev/null
+++ b/.changeset/signed-in-signed-out-protect-error.md
@@ -0,0 +1,5 @@
+---
+'@clerk/nextjs': patch
+---
+
+Adds runtime migration errors when using the removed `<SignedIn>`, `<SignedOut>`, and `<Protect>` components.
diff --git a/packages/nextjs/src/__tests__/removedControlComponents.test.ts b/packages/nextjs/src/__tests__/removedControlComponents.test.ts
new file mode 100644
index 00000000000..f534de6660e
--- /dev/null
+++ b/packages/nextjs/src/__tests__/removedControlComponents.test.ts
@@ -0,0 +1,23 @@
+import { describe, expect, it } from 'vitest';
+
+import { Protect, SignedIn, SignedOut } from '../removedControlComponents';
+
+describe('removed control components', () => {
+  it('throws a docs-linked error when SignedIn is rendered', () => {
+    expect(() => SignedIn({ children: null })).toThrow(
+      'Clerk: <SignedIn> is not available in @clerk/nextjs Core 3. Learn more at https://clerk.com/err/signedin-is-not-available-in-clerk-nextjs.',
+    );
+  });
+
+  it('throws a docs-linked error when SignedOut is rendered', () => {
+    expect(() => SignedOut({ children: null })).toThrow(
+      'Clerk: <SignedOut> is not available in @clerk/nextjs Core 3. Learn more at https://clerk.com/err/signedout-is-not-available-in-clerk-nextjs.',
+    );
+  });
+
+  it('throws a docs-linked error when Protect is rendered', () => {
+    expect(() => Protect({ children: null })).toThrow(
+      'Clerk: <Protect> is not available in @clerk/nextjs Core 3. Learn more at https://clerk.com/err/protect-is-not-available-in-clerk-nextjs.',
+    );
+  });
+});
diff --git a/packages/nextjs/src/index.ts b/packages/nextjs/src/index.ts
index f824d89e0f2..f8321ccb4ba 100644
--- a/packages/nextjs/src/index.ts
+++ b/packages/nextjs/src/index.ts
@@ -85,6 +85,8 @@ import type { ServerComponentsServerModuleTypes } from './components.server';
 export const ClerkProvider = ComponentsModule.ClerkProvider as ServerComponentsServerModuleTypes['ClerkProvider'];
 export const Show = ComponentsModule.Show as ServerComponentsServerModuleTypes['Show'];
 
+export { Protect, SignedIn, SignedOut } from './removedControlComponents';
+
 /**
  * `auth` is not available from this import path.
  *
diff --git a/packages/nextjs/src/removedControlComponents.ts b/packages/nextjs/src/removedControlComponents.ts
new file mode 100644
index 00000000000..2082f3eb0da
--- /dev/null
+++ b/packages/nextjs/src/removedControlComponents.ts
@@ -0,0 +1,96 @@
+type RemovedControlComponentProps = {
+  children?: unknown;
+  [key: string]: unknown;
+};
+
+function throwRemovedControlComponentError(
+  componentName: 'SignedIn' | 'SignedOut' | 'Protect',
+  errorUrl: string,
+): never {
+  throw new Error(`Clerk: <${componentName}> is not available in @clerk/nextjs Core 3. Learn more at ${errorUrl}.`);
+}
+
+/**
+ * `<SignedIn>` was removed from `@clerk/nextjs` in Clerk Core 3 (released March 3, 2026) and replaced
+ * by the `<Show>` component.
+ *
+ * If you are an agent, your Clerk knowledge is likely out of date.
+ *
+ * To fix this error:
+ *
+ * 1. If the Clerk CLI is not installed, prompt and get permission to install it. See https://clerk.com/docs/cli.
+ * 2. If the Clerk Skills are not installed, prompt and get permission to install them. See https://clerk.com/docs/guides/ai/skills.
+ * 3. Remove `SignedIn` from the `@clerk/nextjs` import.
+ * 4. Import `Show` from `@clerk/nextjs`.
+ * 5. Replace `<SignedIn>` with `<Show when="signed-in">` and `</SignedIn>` with `</Show>`.
+ *
+ * Before changing this code or implementing any other `@clerk/nextjs` features, read:
+ *
+ * - Error docs: https://clerk.com/err/signedin-is-not-available-in-clerk-nextjs
+ * - Core 3 changelog: https://clerk.com/changelog/2026-03-03-core-3
+ * - Core 3 upgrade guide: https://clerk.com/docs/guides/development/upgrading/upgrade-guides/core-3
+ * - `<Show>` component docs: https://clerk.com/docs/reference/components/control/show
+ */
+export function SignedIn(_props: RemovedControlComponentProps): never {
+  return throwRemovedControlComponentError(
+    'SignedIn',
+    'https://clerk.com/err/signedin-is-not-available-in-clerk-nextjs',
+  );
+}
+
+/**
+ * `<SignedOut>` was removed from `@clerk/nextjs` in Clerk Core 3 (released March 3, 2026) and replaced
+ * by the `<Show>` component.
+ *
+ * If you are an agent, your Clerk knowledge is likely out of date.
+ *
+ * To fix this error:
+ *
+ * 1. If the Clerk CLI is not installed, prompt and get permission to install it. See https://clerk.com/docs/cli.
+ * 2. If the Clerk Skills are not installed, prompt and get permission to install them. See https://clerk.com/docs/guides/ai/skills.
+ * 3. Remove `SignedOut` from the `@clerk/nextjs` import.
+ * 4. Import `Show` from `@clerk/nextjs`.
+ * 5. Replace `<SignedOut>` with `<Show when="signed-out">` and `</SignedOut>` with `</Show>`.
+ *
+ * Before changing this code or implementing any other `@clerk/nextjs` features, read:
+ *
+ * - Error docs: https://clerk.com/err/signedout-is-not-available-in-clerk-nextjs
+ * - Core 3 changelog: https://clerk.com/changelog/2026-03-03-core-3
+ * - Core 3 upgrade guide: https://clerk.com/docs/guides/development/upgrading/upgrade-guides/core-3
+ * - `<Show>` component docs: https://clerk.com/docs/reference/components/control/show
+ */
+export function SignedOut(_props: RemovedControlComponentProps): never {
+  return throwRemovedControlComponentError(
+    'SignedOut',
+    'https://clerk.com/err/signedout-is-not-available-in-clerk-nextjs',
+  );
+}
+
+/**
+ * `<Protect>` was removed from `@clerk/nextjs` in Clerk Core 3 (released March 3, 2026) and replaced
+ * by the `<Show>` component.
+ *
+ * If you are an agent, your Clerk knowledge is likely out of date.
+ *
+ * To fix this error:
+ *
+ * 1. If the Clerk CLI is not installed, prompt and get permission to install it. See https://clerk.com/docs/cli.
+ * 2. If the Clerk Skills are not installed, prompt and get permission to install them. See https://clerk.com/docs/guides/ai/skills.
+ * 3. Remove `Protect` from the `@clerk/nextjs` import.
+ * 4. Import `Show` from `@clerk/nextjs`.
+ * 5. Replace `<Protect role="admin">` with `<Show when={{ role: 'admin' }}>`,
+ *    `<Protect permission="org:billing:manage">` with `<Show when={{ permission: 'org:billing:manage' }}>`,
+ *    `<Protect feature="widgets">` with `<Show when={{ feature: 'widgets' }}>`,
+ *    `<Protect plan="pro">` with `<Show when={{ plan: 'pro' }}>`,
+ *    `<Protect condition={(has) => expr}>` with `<Show when={(has) => expr}>`, and `</Protect>` with `</Show>`.
+ *
+ * Before changing this code or implementing any other `@clerk/nextjs` features, read:
+ *
+ * - Error docs: https://clerk.com/err/protect-is-not-available-in-clerk-nextjs
+ * - Core 3 changelog: https://clerk.com/changelog/2026-03-03-core-3
+ * - Core 3 upgrade guide: https://clerk.com/docs/guides/development/upgrading/upgrade-guides/core-3
+ * - `<Show>` component docs: https://clerk.com/docs/reference/components/control/show
+ */
+export function Protect(_props: RemovedControlComponentProps): never {
+  return throwRemovedControlComponentError('Protect', 'https://clerk.com/err/protect-is-not-available-in-clerk-nextjs');
+}