diff --git a/CHANGELOG.md b/CHANGELOG.md
index f2160d1fa9b..27b179a7f3b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -37,6 +37,17 @@
 - Removed `apollo-boost` since Apollo Client 3.0 provides a boost like getting started experience out of the box.  <br/>
   [@hwillson](https://github.com/hwillson) in [#5217](https://github.com/apollographql/apollo-client/pull/5217)
 
+- `InMemoryCache` provides a new API for storing local state that can be easily updated by external code:
+  ```ts
+  const lv = cache.makeLocalVar(123)
+  console.log(lv()) // 123
+  console.log(lv(lv() + 1)) // 124
+  console.log(lv()) // 124
+  lv("asdf") // TS type error
+  ```
+  These local variables are _reactive_ in the sense that updating their values invalidates any previously cached query results that depended on the old values. <br/>
+  [@benjamn](https://github.com/benjamn) in [#5799](https://github.com/apollographql/apollo-client/pull/5799)
+
 - The `queryManager` property of `ApolloClient` instances is now marked as
   `private`, paving the way for a more aggressive redesign of its API.
 
diff --git a/src/cache/inmemory/__tests__/cache.ts b/src/cache/inmemory/__tests__/cache.ts
index d4a59790acf..e35469d9960 100644
--- a/src/cache/inmemory/__tests__/cache.ts
+++ b/src/cache/inmemory/__tests__/cache.ts
@@ -1324,3 +1324,110 @@ describe("InMemoryCache#broadcastWatches", function () {
     ]);
   });
 });
+
+describe("cache.makeLocalVar", () => {
+  function makeCacheAndVar(resultCaching: boolean) {
+    const cache: InMemoryCache = new InMemoryCache({
+      resultCaching,
+      typePolicies: {
+        Person: {
+          fields: {
+            name() {
+              return nameVar();
+            },
+          },
+        },
+      },
+    });
+
+    const nameVar = cache.makeLocalVar("Ben");
+
+    const query = gql`
+      query {
+        onCall {
+          name
+        }
+      }
+    `;
+
+    cache.writeQuery({
+      query,
+      data: {
+        onCall: {
+          __typename: "Person",
+        },
+      },
+    });
+
+    return {
+      cache,
+      nameVar,
+      query,
+    };
+  }
+
+  it("should work with resultCaching enabled (default)", () => {
+    const { cache, nameVar, query } = makeCacheAndVar(true);
+
+    const result1 = cache.readQuery({ query });
+    expect(result1).toEqual({
+      onCall: {
+        __typename: "Person",
+        name: "Ben",
+      },
+    });
+
+    // No change before updating the nameVar.
+    expect(cache.readQuery({ query })).toBe(result1);
+
+    expect(nameVar()).toBe("Ben");
+    expect(nameVar("Hugh")).toBe("Hugh");
+
+    const result2 = cache.readQuery({ query });
+    expect(result2).not.toBe(result1);
+    expect(result2).toEqual({
+      onCall: {
+        __typename: "Person",
+        name: "Hugh",
+      },
+    });
+
+    expect(nameVar()).toBe("Hugh");
+    expect(nameVar("James")).toBe("James");
+
+    expect(cache.readQuery({ query })).toEqual({
+      onCall: {
+        __typename: "Person",
+        name: "James",
+      },
+    });
+  });
+
+  it("should work with resultCaching disabled (unusual)", () => {
+    const { cache, nameVar, query } = makeCacheAndVar(false);
+
+    const result1 = cache.readQuery({ query });
+    expect(result1).toEqual({
+      onCall: {
+        __typename: "Person",
+        name: "Ben",
+      },
+    });
+
+    const result2 = cache.readQuery({ query });
+    // Without resultCaching, equivalent results will not be ===.
+    expect(result2).not.toBe(result1);
+    expect(result2).toEqual(result1);
+
+    expect(nameVar()).toBe("Ben");
+    expect(nameVar("Hugh")).toBe("Hugh");
+
+    const result3 = cache.readQuery({ query });
+    expect(result3).toEqual({
+      onCall: {
+        __typename: "Person",
+        name: "Hugh",
+      },
+    });
+  });
+});
diff --git a/src/cache/inmemory/inMemoryCache.ts b/src/cache/inmemory/inMemoryCache.ts
index c139e43e5ad..82d8c96a9e6 100644
--- a/src/cache/inmemory/inMemoryCache.ts
+++ b/src/cache/inmemory/inMemoryCache.ts
@@ -2,7 +2,7 @@
 import './fixPolyfills';
 
 import { DocumentNode } from 'graphql';
-import { wrap } from 'optimism';
+import { dep, wrap } from 'optimism';
 
 import { ApolloCache, Transaction } from '../core/cache';
 import { Cache } from '../core/types/Cache';
@@ -298,4 +298,21 @@ export class InMemoryCache extends ApolloCache<NormalizedCacheObject> {
       }),
     );
   }
+
+  public makeLocalVar<T>(value: T): LocalVar<T> {
+    return function LocalVar(newValue) {
+      if (arguments.length > 0) {
+        if (value !== newValue) {
+          value = newValue;
+          localVarDep.dirty(LocalVar);
+        }
+      } else {
+        localVarDep(LocalVar);
+      }
+      return value;
+    };
+  }
 }
+
+const localVarDep = dep<LocalVar<any>>();
+export type LocalVar<T> = (newValue?: T) => T;