diff --git a/components/constellation/constellation.rs b/components/constellation/constellation.rs
index 9eef83f33a08..209094823b5a 100644
--- a/components/constellation/constellation.rs
+++ b/components/constellation/constellation.rs
@@ -1562,10 +1562,17 @@ where
             FromScriptMsg::PostMessage {
                 target: browsing_context_id,
                 source: source_pipeline_id,
+                source_origin: source_origin,
                 target_origin: origin,
                 data,
             } => {
-                self.handle_post_message_msg(browsing_context_id, source_pipeline_id, origin, data);
+                self.handle_post_message_msg(
+                    browsing_context_id,
+                    source_pipeline_id,
+                    source_origin,
+                    origin,
+                    data,
+                );
             },
             FromScriptMsg::Focus => {
                 self.handle_focus_msg(source_pipeline_id);
@@ -3202,6 +3209,7 @@ where
         &mut self,
         browsing_context_id: BrowsingContextId,
         source_pipeline: PipelineId,
+        source_origin: Option<ImmutableOrigin>,
         origin: Option<ImmutableOrigin>,
         data: Vec<u8>,
     ) {
@@ -3222,6 +3230,7 @@ where
             target: pipeline_id,
             source: source_pipeline,
             source_browsing_context: source_browsing_context,
+            source_origin: source_origin,
             target_origin: origin,
             data,
         };
diff --git a/components/script/dom/dissimilaroriginwindow.rs b/components/script/dom/dissimilaroriginwindow.rs
index 8f7eda581006..197bf0fb4b18 100644
--- a/components/script/dom/dissimilaroriginwindow.rs
+++ b/components/script/dom/dissimilaroriginwindow.rs
@@ -152,8 +152,13 @@ impl DissimilarOriginWindowMethods for DissimilarOriginWindow {
         // TODO(#12717): Should implement the `transfer` argument.
         let data = StructuredCloneData::write(*cx, message)?;
 
+        let source_origin = match self.window_proxy().document() {
+            Some(document) => Some(document.origin().immutable().clone()),
+            None => None,
+        };
+
         // Step 9.
-        self.post_message(origin, data);
+        self.post_message(source_origin, origin, data);
         Ok(())
     }
 
@@ -186,7 +191,12 @@ impl DissimilarOriginWindowMethods for DissimilarOriginWindow {
 }
 
 impl DissimilarOriginWindow {
-    pub fn post_message(&self, origin: Option<ImmutableOrigin>, data: StructuredCloneData) {
+    pub fn post_message(
+        &self,
+        source_origin: Option<ImmutableOrigin>,
+        origin: Option<ImmutableOrigin>,
+        data: StructuredCloneData,
+    ) {
         let incumbent = match GlobalScope::incumbent() {
             None => return warn!("postMessage called with no incumbent global"),
             Some(incumbent) => incumbent,
@@ -194,6 +204,7 @@ impl DissimilarOriginWindow {
         let msg = ScriptMsg::PostMessage {
             target: self.window_proxy.browsing_context_id(),
             source: incumbent.pipeline_id(),
+            source_origin,
             target_origin: origin,
             data: data.move_to_arraybuffer(),
         };
diff --git a/components/script/dom/window.rs b/components/script/dom/window.rs
index a79669d6c2e1..5952aa74735d 100644
--- a/components/script/dom/window.rs
+++ b/components/script/dom/window.rs
@@ -915,6 +915,7 @@ impl WindowMethods for Window {
     fn PostMessage(&self, cx: JSContext, message: HandleValue, origin: DOMString) -> ErrorResult {
         let source_global = GlobalScope::incumbent().expect("no incumbent global??");
         let source = source_global.as_window();
+        let source_origin = Some(source.Document().origin().immutable().clone());
 
         // Step 3-5.
         let origin = match &origin[..] {
@@ -931,7 +932,7 @@ impl WindowMethods for Window {
         let data = StructuredCloneData::write(*cx, message)?;
 
         // Step 9.
-        self.post_message(origin, &*source.window_proxy(), data);
+        self.post_message(source_origin, origin, &*source.window_proxy(), data);
         Ok(())
     }
 
@@ -2269,12 +2270,14 @@ impl Window {
     // https://html.spec.whatwg.org/multipage/#dom-window-postmessage step 7.
     pub fn post_message(
         &self,
+        source_origin: Option<ImmutableOrigin>,
         target_origin: Option<ImmutableOrigin>,
         source: &WindowProxy,
         serialize_with_transfer_result: StructuredCloneData,
     ) {
         let this = Trusted::new(self);
         let source = Trusted::new(source);
+        let origin = source_origin.map(|origin| origin.ascii_serialization());
         let task = task!(post_serialised_message: move || {
             let this = this.root();
             let source = source.root();
@@ -2305,7 +2308,7 @@ impl Window {
                 this.upcast(),
                 this.upcast(),
                 message_clone.handle(),
-                None,
+                origin.as_ref().map( |origin| { &**origin } ),
                 Some(&*source),
             );
         });
diff --git a/components/script/script_thread.rs b/components/script/script_thread.rs
index 4cda69cdcedc..1f9f056727b7 100644
--- a/components/script/script_thread.rs
+++ b/components/script/script_thread.rs
@@ -1838,12 +1838,14 @@ impl ScriptThread {
                 target: target_pipeline_id,
                 source: source_pipeline_id,
                 source_browsing_context,
+                source_origin,
                 target_origin: origin,
                 data,
             } => self.handle_post_message_msg(
                 target_pipeline_id,
                 source_pipeline_id,
                 source_browsing_context,
+                source_origin,
                 origin,
                 data,
             ),
@@ -2488,6 +2490,7 @@ impl ScriptThread {
         pipeline_id: PipelineId,
         source_pipeline_id: PipelineId,
         source_browsing_context: TopLevelBrowsingContextId,
+        source_origin: Option<ImmutableOrigin>,
         origin: Option<ImmutableOrigin>,
         data: Vec<u8>,
     ) {
@@ -2511,7 +2514,12 @@ impl ScriptThread {
                     Some(source) => source,
                 };
                 // FIXME(#22512): enqueues a task; unnecessary delay.
-                window.post_message(origin, &*source, StructuredCloneData::Vector(data))
+                window.post_message(
+                    source_origin,
+                    origin,
+                    &*source,
+                    StructuredCloneData::Vector(data),
+                )
             },
         }
     }
diff --git a/components/script_traits/lib.rs b/components/script_traits/lib.rs
index 1aa37e565337..19f9e34cfd6c 100644
--- a/components/script_traits/lib.rs
+++ b/components/script_traits/lib.rs
@@ -322,6 +322,8 @@ pub enum ConstellationControlMsg {
         source: PipelineId,
         /// The top level browsing context associated with the source pipeline.
         source_browsing_context: TopLevelBrowsingContextId,
+        /// Source origin of the message
+        source_origin: Option<ImmutableOrigin>,
         /// The expected origin of the target.
         target_origin: Option<ImmutableOrigin>,
         /// The data to be posted.
diff --git a/components/script_traits/script_msg.rs b/components/script_traits/script_msg.rs
index c3ee8d752054..54d06dac9dc5 100644
--- a/components/script_traits/script_msg.rs
+++ b/components/script_traits/script_msg.rs
@@ -164,6 +164,8 @@ pub enum ScriptMsg {
         target: BrowsingContextId,
         /// The source of the posted message.
         source: PipelineId,
+        /// Source origin of the message
+        source_origin: Option<ImmutableOrigin>,
         /// The expected origin of the target.
         target_origin: Option<ImmutableOrigin>,
         /// The data to be posted.