diff --git a/packages/query-core/src/__tests__/utils.test.tsx b/packages/query-core/src/__tests__/utils.test.tsx
index 064795279b2..fdecf0d9923 100644
--- a/packages/query-core/src/__tests__/utils.test.tsx
+++ b/packages/query-core/src/__tests__/utils.test.tsx
@@ -73,6 +73,14 @@ describe('core/utils', () => {
 
       expect(isPlainObject(Object.create(Graph))).toBeFalsy()
     })
+
+    it('should return `false` for object with custom prototype', () => {
+      const CustomProto = Object.create({ a: 1 })
+      const obj = Object.create(CustomProto)
+      obj.b = 2
+
+      expect(isPlainObject(obj)).toBeFalsy()
+    })
   })
 
   describe('isPlainArray', () => {
diff --git a/packages/query-core/src/utils.ts b/packages/query-core/src/utils.ts
index c100ac4baa0..251b55cba1b 100644
--- a/packages/query-core/src/utils.ts
+++ b/packages/query-core/src/utils.ts
@@ -300,6 +300,11 @@ export function isPlainObject(o: any): o is Object {
     return false
   }
 
+  // Handles Objects created by Object.create(<arbitrary prototype>)
+  if (Object.getPrototypeOf(o) !== Object.prototype) {
+    return false
+  }
+
   // Most likely a plain Object
   return true
 }