diff --git a/CHANGELOG.md b/CHANGELOG.md
index 288ab54ac97..052c6689689 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -70,6 +70,9 @@
 - Cancel `queryInfo.notifyTimeout` in `QueryInfo#markResult` to prevent unnecessary network requests when using a `FetchPolicy` of `cache-and-network` or `network-only` in a React component with multiple `useQuery` calls. <br/>
   [@benjamn](https://github.com/benjamn) in [#7347](https://github.com/apollographql/apollo-client/pull/7347)
 
+- `ApolloCache` objects (including `InMemoryCache`) may now be associated with or disassociated from individual reactive variables by calling `reactiveVar.attachCache(cache)` and/or `reactiveVar.forgetCache(cache)`. <br/>
+  [@benjamn](https://github.com/benjamn) in [#7350](https://github.com/apollographql/apollo-client/pull/7350)
+
 ## Apollo Client 3.2.7
 
 ## Bug Fixes
diff --git a/package.json b/package.json
index e6d96b13999..c669d22fa26 100644
--- a/package.json
+++ b/package.json
@@ -53,7 +53,7 @@
     {
       "name": "apollo-client",
       "path": "./dist/apollo-client.cjs.min.js",
-      "maxSize": "25.4 kB"
+      "maxSize": "25.5 kB"
     }
   ],
   "peerDependencies": {
diff --git a/src/cache/inmemory/__tests__/cache.ts b/src/cache/inmemory/__tests__/cache.ts
index 212afe8bcc6..f58542e4449 100644
--- a/src/cache/inmemory/__tests__/cache.ts
+++ b/src/cache/inmemory/__tests__/cache.ts
@@ -2056,7 +2056,6 @@ describe("InMemoryCache#modify", () => {
     cache.modify({
       fields: {
         comments(comments: Reference[], { readField }) {
-          debugger;
           expect(Object.isFrozen(comments)).toBe(true);
           expect(comments.length).toBe(3);
           const filtered = comments.filter(comment => {
@@ -2757,6 +2756,133 @@ describe("ReactiveVar and makeVar", () => {
       },
     ]);
   });
+
+  it('should broadcast to manually added caches', () => {
+    const rv = makeVar(0);
+    const cache = new InMemoryCache;
+    const query = gql`query { value }`;
+    const diffs: Cache.DiffResult<any>[] = [];
+    const watch: Cache.WatchOptions = {
+      query,
+      optimistic: true,
+      callback(diff) {
+        diffs.push(diff);
+      },
+    };
+
+    cache.writeQuery({
+      query,
+      data: {
+        value: "oyez",
+      },
+    });
+
+    const cancel = cache.watch(watch);
+
+    // This should not trigger a broadcast, since we haven't associated
+    // this cache with rv yet.
+    rv(rv() + 1);
+    expect(diffs).toEqual([]);
+
+    // The rv.attachCache method returns rv, for chaining.
+    rv.attachCache(cache)(rv() + 1);
+
+    expect(diffs).toEqual([
+      {
+        complete: true,
+        result: {
+          value: "oyez",
+        },
+      },
+    ]);
+
+    cache.writeQuery({
+      query,
+      broadcast: false,
+      data: {
+        value: "oyez, oyez",
+      },
+    });
+
+    expect(diffs).toEqual([
+      {
+        complete: true,
+        result: {
+          value: "oyez",
+        },
+      },
+    ]);
+
+    rv(rv() + 1);
+    expect(diffs).toEqual([
+      {
+        complete: true,
+        result: {
+          value: "oyez",
+        },
+      },
+      {
+        complete: true,
+        result: {
+          value: "oyez, oyez",
+        },
+      },
+    ]);
+
+    expect(rv.forgetCache(cache)).toBe(true);
+
+    cache.writeQuery({
+      query,
+      broadcast: false,
+      data: {
+        value: "oyez, oyez, oyez",
+      },
+    });
+
+    // Since we called rv.forgetCache(cache) above, updating rv here
+    // should not trigger a broadcast.
+    rv(rv() + 1);
+    expect(diffs).toEqual([
+      {
+        complete: true,
+        result: {
+          value: "oyez",
+        },
+      },
+      {
+        complete: true,
+        result: {
+          value: "oyez, oyez",
+        },
+      },
+    ]);
+
+    cache["broadcastWatches"]();
+    expect(diffs).toEqual([
+      {
+        complete: true,
+        result: {
+          value: "oyez",
+        },
+      },
+      {
+        complete: true,
+        result: {
+          value: "oyez, oyez",
+        },
+      },
+      {
+        complete: true,
+        result: {
+          value: "oyez, oyez, oyez",
+        },
+      },
+    ]);
+
+    cancel();
+
+    expect(rv()).toBe(4);
+  });
 });
 
 describe('TypedDocumentNode<Data, Variables>', () => {
diff --git a/src/cache/inmemory/reactiveVars.ts b/src/cache/inmemory/reactiveVars.ts
index d7b95c62832..409c8611837 100644
--- a/src/cache/inmemory/reactiveVars.ts
+++ b/src/cache/inmemory/reactiveVars.ts
@@ -6,6 +6,7 @@ import { ApolloCache } from '../../core';
 export interface ReactiveVar<T> {
   (newValue?: T): T;
   onNextChange(listener: ReactiveListener<T>): () => void;
+  attachCache(cache: ApolloCache<any>): this;
   forgetCache(cache: ApolloCache<any>): boolean;
 }
 
@@ -22,10 +23,12 @@ export const cacheSlot = new Slot<ApolloCache<any>>();
 // to the original elements of the set before we begin iterating. See
 // iterateObserversSafely for another example of this pattern.
 function consumeAndIterate<T>(set: Set<T>, callback: (item: T) => any) {
-  const items: T[] = [];
-  set.forEach(item => items.push(item));
-  set.clear();
-  items.forEach(callback);
+  if (set.size) {
+    const items: T[] = [];
+    set.forEach(item => items.push(item));
+    set.clear();
+    items.forEach(callback);
+  }
 }
 
 const varsByCache = new WeakMap<ApolloCache<any>, Set<ReactiveVar<any>>>();
@@ -61,12 +64,7 @@ export function makeVar<T>(value: T): ReactiveVar<T> {
       // context via cacheSlot. This isn't entirely foolproof, but it's
       // the same system that powers varDep.
       const cache = cacheSlot.getValue();
-      if (cache) {
-        caches.add(cache);
-        let vars = varsByCache.get(cache)!;
-        if (!vars) varsByCache.set(cache, vars = new Set);
-        vars.add(rv);
-      }
+      if (cache) attach(cache);
       varDep(rv);
     }
 
@@ -80,7 +78,22 @@ export function makeVar<T>(value: T): ReactiveVar<T> {
     };
   };
 
-  rv.forgetCache = cache => caches.delete(cache);
+  const attach = rv.attachCache = cache => {
+    caches.add(cache);
+    let vars = varsByCache.get(cache)!;
+    if (!vars) varsByCache.set(cache, vars = new Set);
+    vars.add(rv);
+    return rv;
+  };
+
+  rv.forgetCache = cache => {
+    const deleted = caches.delete(cache);
+    if (deleted) {
+      const vars = varsByCache.get(cache);
+      if (vars) vars.delete(rv);
+    }
+    return deleted;
+  };
 
   return rv;
 }