diff --git a/packages/core/src/index.ts b/packages/core/src/index.ts
index becc9c7..62d1392 100644
--- a/packages/core/src/index.ts
+++ b/packages/core/src/index.ts
@@ -531,3 +531,10 @@ export function untrack<T>(fn: () => T): T {
     popSubscriber();
   }
 }
+
+// ---------------------------------------------------------------------------
+// Shared signals (cross-tab sync via BroadcastChannel)
+// ---------------------------------------------------------------------------
+
+export { sharedSignal } from './shared.js';
+export type { SharedSignal, SharedSignalOptions } from './shared.js';
diff --git a/packages/core/src/shared.test.ts b/packages/core/src/shared.test.ts
new file mode 100644
index 0000000..e41a786
--- /dev/null
+++ b/packages/core/src/shared.test.ts
@@ -0,0 +1,209 @@
+// ============================================================================
+// shared.test.ts — Tests for sharedSignal (cross-tab state sync)
+// ============================================================================
+
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import { effect } from './index';
+import { sharedSignal, type SharedSignal } from './shared';
+
+// ---------------------------------------------------------------------------
+// BroadcastChannel mock
+// ---------------------------------------------------------------------------
+
+// Track all channels by name so we can simulate cross-tab messaging.
+const channels: Map<
+  string,
+  Set<{ onmessage: ((e: MessageEvent) => void) | null; postMessage: (data: any) => void }>
+> = new Map();
+
+class MockBroadcastChannel {
+  name: string;
+  onmessage: ((e: MessageEvent) => void) | null = null;
+
+  constructor(name: string) {
+    this.name = name;
+    if (!channels.has(name)) {
+      channels.set(name, new Set());
+    }
+    channels.get(name)!.add(this);
+  }
+
+  postMessage(data: any): void {
+    const group = channels.get(this.name);
+    if (!group) return;
+    for (const ch of group) {
+      if (ch !== this && ch.onmessage) {
+        // Simulate async delivery (like real BroadcastChannel).
+        ch.onmessage(new MessageEvent('message', { data }));
+      }
+    }
+  }
+
+  close(): void {
+    const group = channels.get(this.name);
+    if (group) {
+      group.delete(this);
+      if (group.size === 0) channels.delete(this.name);
+    }
+  }
+}
+
+beforeEach(() => {
+  channels.clear();
+  (globalThis as any).BroadcastChannel = MockBroadcastChannel;
+});
+
+afterEach(() => {
+  channels.clear();
+});
+
+// ---------------------------------------------------------------------------
+// Tests
+// ---------------------------------------------------------------------------
+
+describe('sharedSignal', () => {
+  it('behaves as a normal signal for reads and writes', () => {
+    const s = sharedSignal('test-basic', 0);
+    expect(s()).toBe(0);
+    expect(s.peek()).toBe(0);
+
+    s.set(42);
+    expect(s()).toBe(42);
+
+    s.update((n) => n + 1);
+    expect(s()).toBe(43);
+
+    s.close();
+  });
+
+  it('syncs value from one tab to another', () => {
+    const tab1 = sharedSignal('test-sync', 'hello');
+    const tab2 = sharedSignal('test-sync', 'hello');
+
+    tab1.set('world');
+    expect(tab2()).toBe('world');
+
+    tab1.close();
+    tab2.close();
+  });
+
+  it('syncs in both directions', () => {
+    const tab1 = sharedSignal('test-bidir', 0);
+    const tab2 = sharedSignal('test-bidir', 0);
+
+    tab1.set(10);
+    expect(tab2()).toBe(10);
+
+    tab2.set(20);
+    expect(tab1()).toBe(20);
+
+    tab1.close();
+    tab2.close();
+  });
+
+  it('triggers reactive effects on remote updates', () => {
+    const tab1 = sharedSignal('test-effect', 'a');
+    const tab2 = sharedSignal('test-effect', 'a');
+
+    const values: string[] = [];
+    const dispose = effect(() => {
+      values.push(tab2());
+    });
+
+    expect(values).toEqual(['a']);
+
+    tab1.set('b');
+    expect(values).toEqual(['a', 'b']);
+
+    dispose();
+    tab1.close();
+    tab2.close();
+  });
+
+  it('update() broadcasts the new value', () => {
+    const tab1 = sharedSignal('test-update', 5);
+    const tab2 = sharedSignal('test-update', 5);
+
+    tab1.update((n) => n * 2);
+    expect(tab2()).toBe(10);
+
+    tab1.close();
+    tab2.close();
+  });
+
+  it('does not echo remote updates back to the channel', () => {
+    const tab1 = sharedSignal('test-no-echo', 0);
+    const tab2 = sharedSignal('test-no-echo', 0);
+
+    // Spy on postMessage of tab2's underlying channel.
+    const group = channels.get('utopia:shared:test-no-echo')!;
+    const tab2Channel = Array.from(group)[1];
+    const postSpy = vi.spyOn(tab2Channel, 'postMessage');
+
+    // When tab1 sets, tab2 receives but should NOT re-broadcast.
+    tab1.set(99);
+    expect(tab2()).toBe(99);
+    expect(postSpy).not.toHaveBeenCalled();
+
+    postSpy.mockRestore();
+    tab1.close();
+    tab2.close();
+  });
+
+  it('supports custom serialization', () => {
+    const tab1 = sharedSignal('test-custom', new Date('2025-01-01'), {
+      serialize: (d) => d.toISOString(),
+      deserialize: (s) => new Date(s),
+    });
+    const tab2 = sharedSignal('test-custom', new Date('2000-01-01'), {
+      serialize: (d) => d.toISOString(),
+      deserialize: (s) => new Date(s),
+    });
+
+    tab1.set(new Date('2030-06-15'));
+    expect(tab2().toISOString()).toBe('2030-06-15T00:00:00.000Z');
+
+    tab1.close();
+    tab2.close();
+  });
+
+  it('stops syncing after close()', () => {
+    const tab1 = sharedSignal('test-close', 0);
+    const tab2 = sharedSignal('test-close', 0);
+
+    tab2.close();
+
+    tab1.set(42);
+    // tab2 should NOT have updated since its channel is closed.
+    expect(tab2()).toBe(0);
+
+    tab1.close();
+  });
+
+  it('works as a plain signal when BroadcastChannel is unavailable', () => {
+    // Remove BroadcastChannel to simulate SSR / unsupported env.
+    delete (globalThis as any).BroadcastChannel;
+
+    const s = sharedSignal('test-no-bc', 'hello');
+    expect(s()).toBe('hello');
+    s.set('world');
+    expect(s()).toBe('world');
+
+    // close() should not throw.
+    s.close();
+  });
+
+  it('syncs across 3+ tabs', () => {
+    const tab1 = sharedSignal('test-multi', 0);
+    const tab2 = sharedSignal('test-multi', 0);
+    const tab3 = sharedSignal('test-multi', 0);
+
+    tab1.set(7);
+    expect(tab2()).toBe(7);
+    expect(tab3()).toBe(7);
+
+    tab1.close();
+    tab2.close();
+    tab3.close();
+  });
+});
diff --git a/packages/core/src/shared.ts b/packages/core/src/shared.ts
new file mode 100644
index 0000000..a577522
--- /dev/null
+++ b/packages/core/src/shared.ts
@@ -0,0 +1,112 @@
+// ============================================================================
+// @matthesketh/utopia-core — Shared signals (cross-tab state sync)
+// ============================================================================
+//
+// Provides `sharedSignal()`, a signal that automatically synchronizes its
+// value across browser tabs/windows of the same origin using the
+// BroadcastChannel API.
+//
+// On the server (SSR) or in environments without BroadcastChannel, it
+// falls back to a regular signal with no cross-tab behavior.
+// ============================================================================
+
+import { signal, type Signal } from './index.js';
+
+/**
+ * Options for creating a shared signal.
+ */
+export interface SharedSignalOptions<T> {
+  /** Custom serializer (default: JSON.stringify). */
+  serialize?: (value: T) => string;
+  /** Custom deserializer (default: JSON.parse). */
+  deserialize?: (raw: string) => T;
+}
+
+/**
+ * A shared signal that syncs across browser tabs via BroadcastChannel.
+ * Extends the standard Signal interface with a `close()` method to
+ * tear down the channel.
+ */
+export interface SharedSignal<T> extends Signal<T> {
+  /** Close the BroadcastChannel and stop syncing. */
+  close(): void;
+}
+
+/**
+ * Creates a reactive signal that synchronizes its value across browser
+ * tabs/windows using the BroadcastChannel API.
+ *
+ * ```ts
+ * const theme = sharedSignal('theme', 'light');
+ *
+ * // Setting in one tab updates all other tabs:
+ * theme.set('dark');
+ *
+ * // Clean up when done:
+ * theme.close();
+ * ```
+ *
+ * @param key          A unique channel name for this shared state.
+ * @param initialValue The initial value (used if no other tab has broadcast yet).
+ * @param options      Optional custom serialization.
+ * @returns A SharedSignal with cross-tab sync.
+ */
+export function sharedSignal<T>(
+  key: string,
+  initialValue: T,
+  options?: SharedSignalOptions<T>,
+): SharedSignal<T> {
+  const serialize = options?.serialize ?? JSON.stringify;
+  const deserialize = options?.deserialize ?? JSON.parse;
+
+  const inner = signal<T>(initialValue);
+
+  // No BroadcastChannel (SSR / older browsers) — return a plain signal.
+  if (typeof BroadcastChannel === 'undefined') {
+    return Object.assign(inner, { close: () => {} }) as SharedSignal<T>;
+  }
+
+  const channel = new BroadcastChannel(`utopia:shared:${key}`);
+  let isRemoteUpdate = false;
+
+  // Listen for updates from other tabs.
+  channel.onmessage = (event: MessageEvent) => {
+    try {
+      const value = deserialize(event.data);
+      isRemoteUpdate = true;
+      inner.set(value);
+      isRemoteUpdate = false;
+    } catch {
+      // Ignore malformed messages.
+    }
+  };
+
+  // Wrap .set() to broadcast changes to other tabs.
+  const originalSet = inner.set.bind(inner);
+  const broadcastSet = (newValue: T): void => {
+    originalSet(newValue);
+    // Only broadcast if this was a local change (not a remote update).
+    if (!isRemoteUpdate) {
+      try {
+        channel.postMessage(serialize(newValue));
+      } catch {
+        // Ignore serialization failures.
+      }
+    }
+  };
+
+  // Wrap .update() to go through broadcastSet.
+  const broadcastUpdate = (fn: (current: T) => T): void => {
+    broadcastSet(fn(inner.peek()));
+  };
+
+  // Build the SharedSignal by re-assigning set/update and adding close.
+  const shared = inner as unknown as SharedSignal<T>;
+  (shared as any).set = broadcastSet;
+  (shared as any).update = broadcastUpdate;
+  (shared as any).close = () => {
+    channel.close();
+  };
+
+  return shared;
+}