diff --git a/src/reactivity/reactive.ts b/src/reactivity/reactive.ts
index d08b7c5b..f7ddc8fa 100644
--- a/src/reactivity/reactive.ts
+++ b/src/reactivity/reactive.ts
@@ -135,7 +135,7 @@ export function observe<T>(obj: T): T {
 /**
  * Mock __ob__ for object recursively
  */
-function mockReactivityDeep(obj: any, seen = new Set()) {
+export function mockReactivityDeep(obj: any, seen = new Set()) {
   if (seen.has(obj)) return
 
   def(obj, '__ob__', mockObserver(obj))
diff --git a/src/reactivity/set.ts b/src/reactivity/set.ts
index dfbd40f4..2270c9ec 100644
--- a/src/reactivity/set.ts
+++ b/src/reactivity/set.ts
@@ -1,7 +1,14 @@
 import { AnyObject } from '../types/basic'
 import { getVueConstructor } from '../runtimeContext'
-import { isArray, isPrimitive, isUndef, isValidArrayIndex } from '../utils'
-import { defineAccessControl } from './reactive'
+import {
+  isArray,
+  isPrimitive,
+  isUndef,
+  isValidArrayIndex,
+  isObject,
+  hasOwn,
+} from '../utils'
+import { defineAccessControl, mockReactivityDeep } from './reactive'
 
 /**
  * Set a property on an object. Adds the new property, triggers change
@@ -49,6 +56,11 @@ export function set<T>(target: AnyObject, key: any, val: T): T {
   // IMPORTANT: define access control before trigger watcher
   defineAccessControl(target, key, val)
 
+  // in SSR, there is no __ob__. Mock for reactivity check
+  if (isObject(target[key]) && !hasOwn(target[key], '__ob__')) {
+    mockReactivityDeep(target[key])
+  }
+
   ob.dep.notify()
   return val
 }
diff --git a/test/ssr/ssrReactive.spec.ts b/test/ssr/ssrReactive.spec.ts
index 90ec277a..432244be 100644
--- a/test/ssr/ssrReactive.spec.ts
+++ b/test/ssr/ssrReactive.spec.ts
@@ -145,4 +145,11 @@ describe('SSR Reactive', () => {
       `"RangeError: Maximum call stack size exceeded"`
     ).not.toHaveBeenWarned()
   })
+
+  it('should work on objects sets with set()', () => {
+    const state = ref<any>({})
+    set(state.value, 'a', {})
+
+    expect(isReactive(state.value.a)).toBe(true)
+  })
 })