diff --git a/packages/ddp-server/livedata_server.js b/packages/ddp-server/livedata_server.js
index 6ec03e8dda..2a0ec3e39a 100644
--- a/packages/ddp-server/livedata_server.js
+++ b/packages/ddp-server/livedata_server.js
@@ -1095,23 +1095,33 @@ Object.assign(Subscription.prototype, {
   // removed messages for the published objects; if that is necessary, call
   // _removeAllDocuments first.
   _deactivate: function() {
-    var self = this;
-    if (self._deactivated)
+    if (this._deactivated)
       return;
-    self._deactivated = true;
-    self._callStopCallbacks();
+    this._deactivated = true;
+    this._callStopCallbacks().then(() => {
+      // Break reference chains to allow GC of the Session and its data.
+      // Without this, deactivated subscriptions retain live references
+      // to the (now-closed) session indefinitely.
+      this._session = null;
+      this._documents = new Map();
+    });
     Package['facts-base'] && Package['facts-base'].Facts.incrementServerFact(
       "livedata", "subscriptions", -1);
   },
 
-  _callStopCallbacks: function () {
-    var self = this;
-    // Tell listeners, so they can clean up
-    var callbacks = self._stopCallbacks;
-    self._stopCallbacks = [];
-    callbacks.forEach(function (callback) {
-      callback();
-    });
+  _callStopCallbacks: async function () {
+    // In Meteor 3, onStop callbacks can be async (e.g. observeHandle.stop()
+    // returns a Promise). We must await each one so that observer teardown
+    // completes before the subscription is considered fully deactivated.
+    const callbacks = this._stopCallbacks;
+    this._stopCallbacks = [];
+    for (const callback of callbacks) {
+      try {
+        await callback();
+      } catch (e) {
+        Meteor._debug("Exception in onStop callback:", e);
+      }
+    }
   },
 
   // Send remove messages for every document.
@@ -1190,8 +1200,7 @@ Object.assign(Subscription.prototype, {
   // destroyed but the deferred call to _deactivateAllSubscriptions hasn't
   // happened yet.
   _isDeactivated: function () {
-    var self = this;
-    return self._deactivated || self._session.inQueue === null;
+    return this._deactivated || !this._session || this._session.inQueue === null;
   },
 
   /**
diff --git a/packages/ddp-server/livedata_server_tests.js b/packages/ddp-server/livedata_server_tests.js
index 48313dcbcb..6dbb189d05 100644
--- a/packages/ddp-server/livedata_server_tests.js
+++ b/packages/ddp-server/livedata_server_tests.js
@@ -968,4 +968,63 @@ Tinytest.addAsync(
       clientConn.disconnect();
     });
   }
+);
+
+// ============================================================================
+// Async onStop cleanup tests (memory leak fix)
+// ============================================================================
+
+const asyncCleanupTracker = {};
+
+Meteor.publish('test_async_onstop_cleanup', function (trackerId) {
+  this.onStop(async function () {
+    await new Promise(resolve => setTimeout(resolve, 50));
+    asyncCleanupTracker[trackerId] = true;
+  });
+  this.ready();
+});
+
+Tinytest.addAsync(
+  'livedata server - async onStop callbacks complete on unsubscribe',
+  async function (test) {
+    const trackerId = Random.id();
+    asyncCleanupTracker[trackerId] = false;
+
+    const { clientConn } = await getTestConnections(test);
+    const sub = clientConn.subscribe('test_async_onstop_cleanup', trackerId);
+    await sleep(100);
+
+    sub.stop();
+    await sleep(200);
+
+    test.isTrue(
+      asyncCleanupTracker[trackerId],
+      'Async onStop callback should have completed'
+    );
+
+    clientConn.disconnect();
+    delete asyncCleanupTracker[trackerId];
+  }
+);
+
+Tinytest.addAsync(
+  'livedata server - async onStop callbacks complete on disconnect',
+  async function (test) {
+    const trackerId = Random.id();
+    asyncCleanupTracker[trackerId] = false;
+
+    const { clientConn } = await getTestConnections(test);
+    clientConn.subscribe('test_async_onstop_cleanup', trackerId);
+    await sleep(100);
+
+    clientConn.disconnect();
+    await sleep(300);
+
+    test.isTrue(
+      asyncCleanupTracker[trackerId],
+      'Async onStop callback should have completed on disconnect'
+    );
+
+    delete asyncCleanupTracker[trackerId];
+  }
 );
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