diff --git a/eventsource/eventsource-onmessage-realm-support.htm b/eventsource/eventsource-onmessage-realm-support.htm
new file mode 100644
index 00000000000000..63e6d012b4d82b
--- /dev/null
+++ b/eventsource/eventsource-onmessage-realm-support.htm
@@ -0,0 +1,2 @@
+<!DOCTYPE html>
+<title>This page is just used to grab an EventSource constructor</title>
diff --git a/eventsource/eventsource-onmessage-realm.htm b/eventsource/eventsource-onmessage-realm.htm
new file mode 100644
index 00000000000000..9bc8c4646a4b7c
--- /dev/null
+++ b/eventsource/eventsource-onmessage-realm.htm
@@ -0,0 +1,25 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>EventSource: message event Realm</title>
+<link rel="help" href="https://html.spec.whatwg.org/multipage/comms.html#dispatchMessage">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+
+<iframe src="eventsource-onmessage-realm-support.htm"></iframe>
+
+<script>
+"use strict";
+
+async_test(t => {
+  window.onload = t.step_func(() => {
+    const source = new frames[0].EventSource("resources/message.py");
+    t.add_cleanup(() => {
+      source.close();
+    });
+
+    source.onmessage = t.step_func_done(e => {
+      assert_equals(e.constructor, frames[0].MessageEvent);
+    });
+  });
+}, "the MessageEvent must be created in the Realm of the EventSource");
+</script>
