diff --git a/Userland/Libraries/LibWeb/Bindings/WindowObject.cpp b/Userland/Libraries/LibWeb/Bindings/WindowObject.cpp
index cf9ae65202..4ee9eea972 100644
--- a/Userland/Libraries/LibWeb/Bindings/WindowObject.cpp
+++ b/Userland/Libraries/LibWeb/Bindings/WindowObject.cpp
@@ -136,6 +136,13 @@ Origin WindowObject::origin() const
     return impl().associated_document().origin();
 }
 
+// https://heycam.github.io/webidl/#platform-object-setprototypeof
+bool WindowObject::internal_set_prototype_of(JS::Object* prototype)
+{
+    // 1. Return ? SetImmutablePrototype(O, V).
+    return set_immutable_prototype(prototype);
+}
+
 static DOM::Window* impl_from(JS::VM& vm, JS::GlobalObject& global_object)
 {
     // Since this is a non built-in function we must treat it as non-strict mode
diff --git a/Userland/Libraries/LibWeb/Bindings/WindowObject.h b/Userland/Libraries/LibWeb/Bindings/WindowObject.h
index a148d95ee4..eda6c5d72a 100644
--- a/Userland/Libraries/LibWeb/Bindings/WindowObject.h
+++ b/Userland/Libraries/LibWeb/Bindings/WindowObject.h
@@ -60,6 +60,8 @@ public:
         return *constructor;
     }
 
+    virtual bool internal_set_prototype_of(JS::Object* prototype) override;
+
 private:
     virtual void visit_edges(Visitor&) override;