diff --git a/.changeset/smart-oranges-vanish.md b/.changeset/smart-oranges-vanish.md
new file mode 100644
index 000000000000..9fe519ef4f67
--- /dev/null
+++ b/.changeset/smart-oranges-vanish.md
@@ -0,0 +1,12 @@
+---
+'@data-client/core': patch
+'@data-client/react': patch
+---
+
+Allow ctrl.set() value to be a function
+
+```ts
+ctrl.set(Article, { id: 5 }, (article) => ({ id: article.id, votes: article.votes + 1 }))
+```
+
+Note: the response must include values sufficient to compute Entity.pk()
\ No newline at end of file
diff --git a/packages/core/src/actions.ts b/packages/core/src/actions.ts
index 1a181ea80d98..ced5555443b6 100644
--- a/packages/core/src/actions.ts
+++ b/packages/core/src/actions.ts
@@ -40,7 +40,7 @@ export interface SetAction<S extends Queryable = any> {
   type: typeof SET_TYPE;
   schema: S;
   meta: SetMeta;
-  value: Denormalize<S>;
+  value: {} | ((previousValue: Denormalize<S>) => {});
 }
 
 /* setResponse */
diff --git a/packages/core/src/controller/Controller.ts b/packages/core/src/controller/Controller.ts
index c6ec6c8ec7e6..3de12f7aa3be 100644
--- a/packages/core/src/controller/Controller.ts
+++ b/packages/core/src/controller/Controller.ts
@@ -186,18 +186,28 @@ export default class Controller<
    * Sets value for the Queryable and args.
    * @see https://dataclient.io/docs/api/Controller#set
    */
-  set = <S extends Queryable>(
+  set<S extends Queryable>(
+    schema: S,
+    ...rest: readonly [...SchemaArgs<S>, (previousValue: Denormalize<S>) => {}]
+  ): Promise<void>;
+
+  set<S extends Queryable>(
+    schema: S,
+    ...rest: readonly [...SchemaArgs<S>, {}]
+  ): Promise<void>;
+
+  set<S extends Queryable>(
     schema: S,
     ...rest: readonly [...SchemaArgs<S>, any]
-  ): Promise<void> => {
-    const value: Denormalize<S> = rest[rest.length - 1];
+  ): Promise<void> {
+    const value = rest[rest.length - 1];
     const action = createSet(schema, {
       args: rest.slice(0, rest.length - 1) as SchemaArgs<S>,
       value,
     });
     // TODO: reject with error if this fails in reducer
     return this.dispatch(action);
-  };
+  }
 
   /**
    * Sets response for the Endpoint and args.
diff --git a/packages/core/src/controller/createSet.ts b/packages/core/src/controller/createSet.ts
index e57ce5cf4f51..81892e7a1e1b 100644
--- a/packages/core/src/controller/createSet.ts
+++ b/packages/core/src/controller/createSet.ts
@@ -1,4 +1,8 @@
-import type { Queryable, SchemaArgs } from '@data-client/normalizr';
+import type {
+  Denormalize,
+  Queryable,
+  SchemaArgs,
+} from '@data-client/normalizr';
 
 import ensurePojo from './ensurePojo.js';
 import { SET_TYPE } from '../actionTypes.js';
@@ -12,7 +16,7 @@ export default function createSet<S extends Queryable>(
     value,
   }: {
     args: readonly [...SchemaArgs<S>];
-    value: any;
+    value: {} | ((previousValue: Denormalize<S>) => {});
     fetchedAt?: number;
   },
 ): SetAction<S> {
diff --git a/packages/core/src/state/__tests__/reducer.ts b/packages/core/src/state/__tests__/reducer.ts
index f93aca98c135..ad35c0a6486b 100644
--- a/packages/core/src/state/__tests__/reducer.ts
+++ b/packages/core/src/state/__tests__/reducer.ts
@@ -1,4 +1,4 @@
-import { INVALID } from '@data-client/endpoint';
+import { INVALID, Entity } from '@data-client/endpoint';
 import { ArticleResource, Article, PaginatedArticle } from '__tests__/new';
 
 import { Controller } from '../..';
@@ -202,6 +202,75 @@ describe('reducer', () => {
     });
   });
 
+  it('set(function) should do nothing when entity does not exist', () => {
+    const id = 20;
+    const value = (previous: { counter: number }) => ({
+      counter: previous.counter + 1,
+    });
+    class Counter extends Entity {
+      id = 0;
+      counter = 0;
+      pk() {
+        return this.id;
+      }
+
+      static key = 'Counter';
+    }
+    const action: SetAction = {
+      type: SET_TYPE,
+      value,
+      schema: Counter,
+      meta: {
+        args: [{ id }],
+        date: 0,
+        fetchedAt: 0,
+        expiresAt: 1000000000000,
+      },
+    };
+    const newState = reducer(initialState, action);
+    expect(newState).toBe(initialState);
+  });
+
+  it('set(function) should increment when it is found', () => {
+    const id = 20;
+    const value = (previous: { id: number; counter: number }) => ({
+      id: previous.id,
+      counter: previous.counter + 1,
+    });
+    class Counter extends Entity {
+      id = 0;
+      counter = 0;
+      pk() {
+        return this.id;
+      }
+
+      static key = 'Counter';
+    }
+    const action: SetAction = {
+      type: SET_TYPE,
+      value,
+      schema: Counter,
+      meta: {
+        args: [{ id }],
+        date: 0,
+        fetchedAt: 0,
+        expiresAt: 1000000000000,
+      },
+    };
+    const state = {
+      ...initialState,
+      entities: {
+        [Counter.key]: {
+          [id]: { id, counter: 5 },
+        },
+      },
+    };
+    const newState = reducer(state, action);
+    // eslint-disable-next-line @typescript-eslint/ban-ts-comment
+    // @ts-ignore
+    expect(newState.entities[Counter.key]?.[id]?.counter).toBe(6);
+  });
+
   it('set should add entity when it does not exist', () => {
     const id = 20;
     const value = { id, title: 'hi', content: 'this is the content' };
diff --git a/packages/core/src/state/reducer/createReducer.ts b/packages/core/src/state/reducer/createReducer.ts
index 24ca661de96d..19a029af4001 100644
--- a/packages/core/src/state/reducer/createReducer.ts
+++ b/packages/core/src/state/reducer/createReducer.ts
@@ -44,7 +44,7 @@ export default function createReducer(controller: Controller): ReducerType {
         return setResponseReducer(state, action, controller);
 
       case SET_TYPE:
-        return setReducer(state, action);
+        return setReducer(state, action, controller);
 
       case INVALIDATEALL_TYPE:
       case INVALIDATE_TYPE:
diff --git a/packages/core/src/state/reducer/setReducer.ts b/packages/core/src/state/reducer/setReducer.ts
index cd0ab20c9e3a..b16224ae9b45 100644
--- a/packages/core/src/state/reducer/setReducer.ts
+++ b/packages/core/src/state/reducer/setReducer.ts
@@ -1,11 +1,28 @@
 import { normalize } from '@data-client/normalizr';
 
+import Controller from '../../controller/Controller.js';
 import type { State, SetAction } from '../../types.js';
 
-export function setReducer(state: State<unknown>, action: SetAction) {
+export function setReducer(
+  state: State<unknown>,
+  action: SetAction,
+  controller: Controller,
+) {
+  let value: any;
+  if (typeof action.value === 'function') {
+    const previousValue = controller.get(
+      action.schema,
+      ...action.meta.args,
+      state,
+    );
+    if (previousValue === undefined) return state;
+    value = action.value(previousValue);
+  } else {
+    value = action.value;
+  }
   try {
     const { entities, indexes, entityMeta } = normalize(
-      action.value,
+      value,
       action.schema,
       action.meta.args as any,
       state.entities,
diff --git a/website/src/components/Playground/editor-types/@data-client/core.d.ts b/website/src/components/Playground/editor-types/@data-client/core.d.ts
index 274f9d586172..c1bf62a170b5 100644
--- a/website/src/components/Playground/editor-types/@data-client/core.d.ts
+++ b/website/src/components/Playground/editor-types/@data-client/core.d.ts
@@ -293,7 +293,7 @@ interface SetAction<S extends Queryable = any> {
     type: typeof SET_TYPE;
     schema: S;
     meta: SetMeta;
-    value: Denormalize<S>;
+    value: {} | ((previousValue: Denormalize<S>) => {});
 }
 interface SetResponseMeta {
     args: readonly any[];
@@ -508,7 +508,8 @@ declare class Controller<D extends GenericDispatch = DataClientDispatch> {
      * Sets value for the Queryable and args.
      * @see https://dataclient.io/docs/api/Controller#set
      */
-    set: <S extends Queryable>(schema: S, ...rest: readonly [...SchemaArgs<S>, any]) => Promise<void>;
+    set<S extends Queryable>(schema: S, ...rest: readonly [...SchemaArgs<S>, (previousValue: Denormalize<S>) => {}]): Promise<void>;
+    set<S extends Queryable>(schema: S, ...rest: readonly [...SchemaArgs<S>, {}]): Promise<void>;
     /**
      * Sets response for the Endpoint and args.
      * @see https://dataclient.io/docs/api/Controller#setResponse
@@ -710,7 +711,7 @@ declare function createFetch<E extends EndpointInterface & {
 
 declare function createSet<S extends Queryable>(schema: S, { args, fetchedAt, value, }: {
     args: readonly [...SchemaArgs<S>];
-    value: any;
+    value: {} | ((previousValue: Denormalize<S>) => {});
     fetchedAt?: number;
 }): SetAction<S>;