diff --git a/packages/reactivity-core/CHANGELOG.md b/packages/reactivity-core/CHANGELOG.md
index 6a736d5..69bd45b 100644
--- a/packages/reactivity-core/CHANGELOG.md
+++ b/packages/reactivity-core/CHANGELOG.md
@@ -2,8 +2,9 @@
 
 ## v0.4.3 (Unreleased)
 
+-   Introduce `synchronized`, a new kind of signal designed to integrate foreign data sources.
 -   Introduce `subtleWatchDirty`, a function that allows one to watch for signal changes without triggering the re-evaluation of the signal.
--   Add missing `forEach()` method to `ReactiveSet` and `ReactiveMap`.
+-   Add missing `forEach` method to `ReactiveSet` and `ReactiveMap`.
 -   Deprecate `syncEffectOnce` (use `subtleWatchDirty` instead).
 
 ## v0.4.2
diff --git a/packages/reactivity-core/ReactiveImpl.test.ts b/packages/reactivity-core/ReactiveImpl.test.ts
index 1b7d593..a2d0308 100644
--- a/packages/reactivity-core/ReactiveImpl.test.ts
+++ b/packages/reactivity-core/ReactiveImpl.test.ts
@@ -1,5 +1,5 @@
 import { describe, expect, it, vi } from "vitest";
-import { batch, computed, external, reactive } from "./ReactiveImpl";
+import { batch, computed, external, reactive, synchronized } from "./ReactiveImpl";
 import { syncEffect } from "./sync";
 
 describe("reactive", () => {
@@ -295,3 +295,166 @@ describe("external", () => {
         expect(count).toBe(2);
     });
 });
+
+describe("synchronized", () => {
+    it("always accesses the data source when not watched", () => {
+        const getter = vi.fn().mockReturnValue(123);
+        const sync = synchronized(getter, () => {
+            throw new Error("not called");
+        });
+        expect(getter).toHaveBeenCalledTimes(0);
+
+        expect(sync.value).toBe(123);
+        expect(getter).toHaveBeenCalledTimes(1);
+
+        expect(sync.value).toBe(123);
+        expect(getter).toHaveBeenCalledTimes(2); // NOT cached
+    });
+
+    it("subscribes to the data source when watched", () => {
+        const getter = vi.fn().mockReturnValue(123);
+        const unsubscribe = vi.fn();
+        const subscribe = vi.fn().mockReturnValue(unsubscribe);
+
+        const sync = synchronized(getter, subscribe);
+        expect(getter).toHaveBeenCalledTimes(0);
+        expect(subscribe).toHaveBeenCalledTimes(0);
+
+        const { destroy } = syncEffect(() => sync.value);
+        expect(getter).toHaveBeenCalledTimes(1);
+        expect(subscribe).toHaveBeenCalledTimes(1);
+
+        const { destroy: destroy2 } = syncEffect(() => sync.value);
+        expect(getter).toHaveBeenCalledTimes(1); // cached
+        expect(subscribe).toHaveBeenCalledTimes(1); // not called again for second active effect
+
+        destroy2();
+        expect(unsubscribe).toHaveBeenCalledTimes(0);
+
+        destroy();
+        expect(unsubscribe).toHaveBeenCalledTimes(1);
+
+        sync.value;
+        expect(getter).toHaveBeenCalledTimes(2); // called again
+    });
+
+    it("notifies watchers when the data source changes", () => {
+        const ds = new DataSource(0);
+        const getter = vi.fn(() => ds.value);
+        const sync = synchronized(getter, (cb) => ds.subscribe(cb));
+
+        // not subscribed yet
+        expect(ds.listener).toBe(undefined);
+
+        // setup effect
+        const spy = vi.fn();
+        syncEffect(() => {
+            spy(sync.value);
+        });
+        expect(spy).toHaveBeenCalledTimes(1);
+        expect(spy).toHaveBeenLastCalledWith(0);
+        expect(ds.listener).toBeDefined();
+        expect(getter).toHaveBeenCalledTimes(1);
+
+        // data is cached
+        sync.value;
+        expect(getter).toHaveBeenCalledTimes(1);
+
+        // update data source
+        ds.value = 1;
+        expect(spy).toHaveBeenCalledTimes(2);
+        expect(spy).toHaveBeenLastCalledWith(1);
+        expect(getter).toHaveBeenCalledTimes(2);
+    });
+
+    // See this comment:
+    // https://github.com/tc39/proposal-signals/issues/237#issuecomment-2346834056
+    //
+    // The values here are different from the linked comment (some got lost).
+    // This is because preact-signals probes the first dependency of `result`
+    // and re-evaluates `sync` one additional time.
+    //
+    // This is probably not a show stopper because the signal just re-validates more often than necessary.
+    // It would be more of a problem if it would cache too aggressively.
+    it("has weird behavior if the data source has side effects", () => {
+        let count = 0;
+        const sync = synchronized(
+            () => count++,
+            () => () => {}
+        );
+        const dep1 = computed(() => sync.value);
+        const dep2 = computed(() => sync.value);
+        const result = computed(() => `${dep1.value},${dep1.value},${dep2.value},${dep2.value}`);
+        expect(result.value).toMatchInlineSnapshot(`"0,2,3,5"`);
+
+        const { destroy } = syncEffect(() => {
+            expect(result.value).toMatchInlineSnapshot(`"6,6,6,6"`);
+        });
+        destroy();
+
+        expect(result.value).toMatchInlineSnapshot(`"10,12,13,15"`);
+    });
+
+    it("does not cache computes across many levels", () => {
+        const getter = vi.fn().mockReturnValue(1);
+        const sync = synchronized(getter, () => {
+            throw new Error("not called");
+        });
+
+        const c1 = computed(() => sync.value);
+        const c2 = computed(() => c1.value);
+        const c3 = computed(() => c2.value);
+        const c4 = computed(() => c3.value + c2.value);
+        expect(c4.value).toBe(2);
+        expect(getter.mock.calls.length).toMatchInlineSnapshot(`2`);
+
+        // High number of re-computations probably due to re-validation of
+        // dependencies (see previous test).
+        // As long as the value is correct, this is not a major problem.
+        getter.mockReturnValue(2);
+        expect(c4.value).toBe(4);
+        expect(getter.mock.calls.length).toMatchInlineSnapshot(`8`);
+
+        getter.mockReturnValue(3);
+        expect(c4.value).toBe(6);
+        expect(getter.mock.calls.length).toMatchInlineSnapshot(`14`);
+    });
+});
+
+class DataSource {
+    #listener: (() => void) | undefined;
+    #value: number;
+
+    constructor(value = 0) {
+        this.#value = value;
+    }
+
+    get listener() {
+        return this.#listener;
+    }
+
+    get value() {
+        return this.#value;
+    }
+
+    set value(value: number) {
+        if (value !== this.#value) {
+            this.#value = value;
+            this.#listener?.();
+        }
+    }
+
+    subscribe(listener: () => void) {
+        if (this.#listener) {
+            throw new Error("Already subscribed");
+        }
+
+        this.#listener = listener;
+        return () => {
+            if (this.#listener !== listener) {
+                throw new Error("Invalid unsubscribe call");
+            }
+            this.#listener = undefined;
+        };
+    }
+}
diff --git a/packages/reactivity-core/ReactiveImpl.ts b/packages/reactivity-core/ReactiveImpl.ts
index e46d0a5..ef2941b 100644
--- a/packages/reactivity-core/ReactiveImpl.ts
+++ b/packages/reactivity-core/ReactiveImpl.ts
@@ -1,5 +1,6 @@
 import {
-    Signal,
+    ReadonlySignal as RawReadonlySignal,
+    Signal as RawSignal,
     batch as rawBatch,
     computed as rawComputed,
     signal as rawSignal,
@@ -9,9 +10,10 @@ import {
     AddBrand,
     AddWritableBrand,
     ExternalReactive,
+    Reactive,
     ReadonlyReactive,
     RemoveBrand,
-    Reactive
+    SubscribeFunc
 } from "./types";
 
 /**
@@ -163,6 +165,67 @@ export function external<T>(compute: () => T, options?: ReactiveOptions<T>): Ext
     return externalReactive as ExternalReactive<T>;
 }
 
+/**
+ * Creates a signal that synchronizes with a foreign data source.
+ *
+ * This kind of signal is useful when integrating another library (DOM APIs, etc.) that does not
+ * use the reactivity system provided by this library.
+ * The major advantage of this API is that it will automatically subscribe to the foreign data source while the signal is actually being used.
+ *
+ * Principles:
+ * - The `getter` function should return the current value from the foreign data source.
+ *   It should be cheap to call and should not have side effects.
+ *   Ideally it performs some caching on its own, but this is not strictly needed.
+ * - The `subscribe` function should implement whatever logic is necessary to listen for changes.
+ *   It receives a callback function that should be called whenever the value changes.
+ *   `subscribe` should return a cleanup function to unsubscribe - it will be called automatically when appropriate.
+ * - When the signal is not watched in some way, accesses to the `getter` are not cached.
+ * - When the signal is being watched (e.g. by an effect), the signal will automatically subscribe to the foreign data source
+ *   and cache the current value until it is informed about a change.
+ * 
+ * Example:
+ * 
+ * ```ts
+ * import { synchronized, watchValue } from "@conterra/reactivity-core";
+ * 
+ * const abortController = new AbortController();
+ * const abortSignal = abortController.signal;
+ * const aborted = synchronized(
+ *     () => abortSignal.aborted,
+ *     (callback) => {
+ *         abortSignal.addEventListener("abort", callback);
+ *         return () => {
+ *             abortSignal.removeEventListener("abort", callback);
+ *         };
+ *     }
+ * );
+ * 
+ * watchValue(
+ *     () => aborted.value,
+ *     (aborted) => {
+ *         console.log("Aborted:", aborted);
+ *     },
+ *     {
+ *         immediate: true
+ *     }
+ * );
+ * 
+ * setTimeout(() => {
+ *     abortController.abort();
+ * }, 1000);
+ * 
+ * // Prints:
+ * // Aborted: false
+ * // Aborted: true
+ * ```
+ *
+ * @group Primitives
+ */
+export function synchronized<T>(getter: () => T, subscribe: SubscribeFunc): ReadonlyReactive<T> {
+    const impl = new SynchronizedReactiveImpl(getter, subscribe);
+    return impl as AddBrand<typeof impl>;
+}
+
 /**
  * Executes a set of reactive updates implemented in `callback`.
  * Effects are delayed until the batch has completed.
@@ -268,9 +331,9 @@ const CUSTOM_EQUALS = Symbol("equals");
 abstract class ReactiveImpl<T>
     implements RemoveBrand<ReadonlyReactive<T> & Reactive<T> & ExternalReactive<T>>
 {
-    private [REACTIVE_SIGNAL]: Signal<T>;
+    private [REACTIVE_SIGNAL]: RawSignal<T>;
 
-    constructor(signal: Signal<T>) {
+    constructor(signal: RawSignal<T>) {
         this[REACTIVE_SIGNAL] = signal;
     }
 
@@ -330,6 +393,94 @@ class WritableReactiveImpl<T> extends ReactiveImpl<T> {
     }
 }
 
+const INVALIDATE_SIGNAL = Symbol("invalidate_signal");
+const IS_WATCHED = Symbol("is_watched");
+const HAS_SCHEDULED_INVALIDATE = Symbol("has_scheduled_invalidate");
+
+/**
+ * Custom signal implementation for "synchronized" values, i.e. values from a foreign source.
+ * The signal automatically subscribes to the foreign source once it becomes watched (it also unsubscribes automatically).
+ *
+ * Although the implementation is based on a `computed` signal, there may not be any caching involved, depending on the state of the signal.
+ *
+ * 1. The signal is not watched: the "computed" is always out of date.
+ *    This is achieved by immediately invalidating the signal itself from within its own body.
+ *    To the raw signal, this looks like a dependency cycle (which is allowed in this case).
+ * 2. The signal is watched: the value is cached until an update event is received; at which point the foreign data source is accessed again.
+ *
+ * See also https://github.com/tc39/proposal-signals/issues/237
+ */
+class SynchronizedReactiveImpl<T> extends ReactiveImpl<T> {
+    [INVALIDATE_SIGNAL] = rawSignal(false);
+    [IS_WATCHED] = false;
+    [HAS_SCHEDULED_INVALIDATE] = false;
+
+    constructor(getter: () => T, subscribe: SubscribeFunc) {
+        const rawSignal = rawComputedWithSubscriptionHook(
+            () => {
+                this[INVALIDATE_SIGNAL].value;
+                if (!this[IS_WATCHED]) {
+                    this.#invalidate();
+                }
+                return rawUntracked(() => getter());
+            },
+            () => {
+                this[IS_WATCHED] = true;
+                const unsubscribe = subscribe(this.#invalidate);
+                return () => {
+                    this[IS_WATCHED] = false;
+                    unsubscribe();
+                    this.#invalidate();
+                };
+            }
+        );
+        super(rawSignal);
+    }
+
+    #invalidate = () => {
+        this[INVALIDATE_SIGNAL].value = !this[INVALIDATE_SIGNAL].peek();
+    };
+}
+
+// Mangled member names. See https://github.com/preactjs/signals/blob/main/mangle.json.
+const _SUBSCRIBE = "S";
+const _UNSUBSCRIBE = "U";
+
+type RawSignalInternals<T> = RawReadonlySignal<T> & {
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    [_SUBSCRIBE](node: any): void;
+
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    [_UNSUBSCRIBE](node: any): void;
+};
+
+// Overrides the subscribe/unsubscribe methods of a signal to allow for custom subscription hooks.
+function rawComputedWithSubscriptionHook<T>(
+    compute: () => T,
+    subscribe: () => () => void
+): RawReadonlySignal<T> {
+    const signal = rawComputed(compute) as RawSignalInternals<T>;
+    const origSubscribe = signal[_SUBSCRIBE];
+    const origUnsubscribe = signal[_UNSUBSCRIBE];
+
+    let subscriptions = 0;
+    let cleanup: (() => void) | undefined;
+    signal[_SUBSCRIBE] = function patchedSubscribe(node: unknown) {
+        origSubscribe.call(this, node);
+        if (subscriptions++ === 0) {
+            cleanup = subscribe();
+        }
+    };
+    signal[_UNSUBSCRIBE] = function patchedUnsubscribe(node: unknown) {
+        origUnsubscribe.call(this, node);
+        if (--subscriptions === 0) {
+            cleanup?.();
+            cleanup = undefined;
+        }
+    };
+    return signal;
+}
+
 function computeWithEquals<T>(compute: () => T, equals: EqualsFunc<T>) {
     let firstExecution = true;
     let currentValue: T;
diff --git a/packages/reactivity-core/async.ts b/packages/reactivity-core/async.ts
index 7ae88e0..c41d880 100644
--- a/packages/reactivity-core/async.ts
+++ b/packages/reactivity-core/async.ts
@@ -69,6 +69,7 @@ export function effect(callback: EffectCallback): CleanupHandle {
     };
 }
 
+// TODO: Replace useEffectOnce with subtleWatchDirty (or a similar approach using raw effects).
 class AsyncEffect {
     private callback: EffectCallback;
     private cleanup: CleanupFunc | void | undefined;
diff --git a/packages/reactivity-core/index.ts b/packages/reactivity-core/index.ts
index 0293046..97f3b7a 100644
--- a/packages/reactivity-core/index.ts
+++ b/packages/reactivity-core/index.ts
@@ -29,7 +29,8 @@ export {
     type WatchCallback,
     type WatchImmediateCallback,
     type WatchOptions,
-    type CleanupHandle
+    type CleanupHandle,
+    type SubscribeFunc
 } from "./types";
 export {
     type EqualsFunc,
@@ -37,6 +38,7 @@ export {
     reactive,
     computed,
     external,
+    synchronized,
     batch,
     untracked,
     getValue,
diff --git a/packages/reactivity-core/types.ts b/packages/reactivity-core/types.ts
index 6e5ccfd..2da270b 100644
--- a/packages/reactivity-core/types.ts
+++ b/packages/reactivity-core/types.ts
@@ -181,3 +181,13 @@ export interface WatchOptions<T> {
      */
     equal?(prev: T, next: T): boolean;
 }
+
+/**
+ * A function that can subscribe to some external data source (e.g. DOM APIs) for updates.
+ *
+ * The function must register the `callback` to be called whenever the data source changes.
+ * The function should return a cleanup function that de-registers the callback again.
+ *
+ * @group Watching
+ */
+export type SubscribeFunc = (callback: () => void) => CleanupFunc;