diff --git a/Userland/Libraries/LibWeb/HTML/Window.cpp b/Userland/Libraries/LibWeb/HTML/Window.cpp
index b613ebe28fd..84222389ccd 100644
--- a/Userland/Libraries/LibWeb/HTML/Window.cpp
+++ b/Userland/Libraries/LibWeb/HTML/Window.cpp
@@ -888,20 +888,6 @@ WindowProxy* Window::parent()
     return current->window_proxy();
 }
 
-// https://html.spec.whatwg.org/multipage/web-messaging.html#window-post-message-steps
-WebIDL::ExceptionOr<void> Window::post_message_impl(JS::Value message, DeprecatedString const&)
-{
-    // FIXME: This is an ad-hoc hack implementation instead, since we don't currently
-    //        have serialization and deserialization of messages.
-    HTML::queue_global_task(HTML::Task::Source::PostedMessage, *this, [this, message] {
-        HTML::MessageEventInit event_init {};
-        event_init.data = message;
-        event_init.origin = "<origin>"_string.release_value_but_fixme_should_propagate_errors();
-        dispatch_event(HTML::MessageEvent::create(realm(), String::from_deprecated_string(HTML::EventNames::message).release_value_but_fixme_should_propagate_errors(), event_init).release_value_but_fixme_should_propagate_errors());
-    });
-    return {};
-}
-
 // https://html.spec.whatwg.org/multipage/structured-data.html#dom-structuredclone
 WebIDL::ExceptionOr<JS::Value> Window::structured_clone_impl(JS::VM& vm, JS::Value message)
 {
@@ -1141,7 +1127,6 @@ WebIDL::ExceptionOr<void> Window::initialize_web_interfaces(Badge<WindowEnvironm
     define_native_function(realm, "matchMedia", match_media, 1, attr);
     define_native_function(realm, "getSelection", get_selection, 0, attr);
 
-    define_native_function(realm, "postMessage", post_message, 1, attr);
     define_native_function(realm, "structuredClone", structured_clone, 1, attr);
 
     define_native_function(realm, "fetch", Bindings::fetch, 1, attr);
@@ -1257,6 +1242,19 @@ Optional<String> Window::prompt(Optional<String> const& message, Optional<String
     return {};
 }
 
+// https://html.spec.whatwg.org/multipage/web-messaging.html#dom-window-postmessage
+void Window::post_message(JS::Value message, String const&)
+{
+    // FIXME: This is an ad-hoc hack implementation instead, since we don't currently
+    //        have serialization and deserialization of messages.
+    HTML::queue_global_task(HTML::Task::Source::PostedMessage, *this, [this, message] {
+        HTML::MessageEventInit event_init {};
+        event_init.data = message;
+        event_init.origin = "<origin>"_string.release_value_but_fixme_should_propagate_errors();
+        dispatch_event(HTML::MessageEvent::create(realm(), String::from_deprecated_string(HTML::EventNames::message).release_value_but_fixme_should_propagate_errors(), event_init).release_value_but_fixme_should_propagate_errors());
+    });
+}
+
 JS_DEFINE_NATIVE_FUNCTION(Window::open)
 {
     auto* impl = TRY(impl_from(vm));
@@ -1866,16 +1864,6 @@ JS_DEFINE_NATIVE_FUNCTION(Window::outer_height_getter)
     return JS::Value(impl->outer_height());
 }
 
-JS_DEFINE_NATIVE_FUNCTION(Window::post_message)
-{
-    auto* impl = TRY(impl_from(vm));
-    auto target_origin = TRY(vm.argument(1).to_deprecated_string(vm));
-    TRY(Bindings::throw_dom_exception_if_needed(vm, [&] {
-        return impl->post_message_impl(vm.argument(0), target_origin);
-    }));
-    return JS::js_undefined();
-}
-
 JS_DEFINE_NATIVE_FUNCTION(Window::structured_clone)
 {
     auto* impl = TRY(impl_from(vm));
diff --git a/Userland/Libraries/LibWeb/HTML/Window.h b/Userland/Libraries/LibWeb/HTML/Window.h
index b1819f22bc0..b88dad92e6c 100644
--- a/Userland/Libraries/LibWeb/HTML/Window.h
+++ b/Userland/Libraries/LibWeb/HTML/Window.h
@@ -119,7 +119,6 @@ public:
     // https://html.spec.whatwg.org/multipage/browsers.html#dom-parent
     WindowProxy* parent();
 
-    WebIDL::ExceptionOr<void> post_message_impl(JS::Value, DeprecatedString const& target_origin);
     WebIDL::ExceptionOr<JS::Value> structured_clone_impl(JS::VM& vm, JS::Value);
 
     DeprecatedString name() const;
@@ -145,6 +144,8 @@ public:
     bool confirm(Optional<String> const& message);
     Optional<String> prompt(Optional<String> const& message, Optional<String> const& default_);
 
+    void post_message(JS::Value message, String const&);
+
 private:
     explicit Window(JS::Realm&);
 
@@ -258,7 +259,6 @@ private:
     JS_DECLARE_NATIVE_FUNCTION(outer_width_getter);
     JS_DECLARE_NATIVE_FUNCTION(outer_height_getter);
 
-    JS_DECLARE_NATIVE_FUNCTION(post_message);
     JS_DECLARE_NATIVE_FUNCTION(structured_clone);
 
     JS_DECLARE_NATIVE_FUNCTION(local_storage_getter);
diff --git a/Userland/Libraries/LibWeb/HTML/Window.idl b/Userland/Libraries/LibWeb/HTML/Window.idl
index f8f891fde74..36989d95655 100644
--- a/Userland/Libraries/LibWeb/HTML/Window.idl
+++ b/Userland/Libraries/LibWeb/HTML/Window.idl
@@ -9,6 +9,10 @@ interface Window : EventTarget {
     undefined alert(DOMString message);
     boolean confirm(optional DOMString message = "");
     DOMString? prompt(optional DOMString message = "", optional DOMString default = "");
+
+    undefined postMessage(any message, USVString targetOrigin);
+    // FIXME: undefined postMessage(any message, USVString targetOrigin, optional sequence<object> transfer = []);
+    // FIXME: undefined postMessage(any message, optional WindowPostMessageOptions options = {});
 };
 Window includes GlobalEventHandlers;
 Window includes WindowEventHandlers;