diff --git a/dom/serviceworkers/PServiceWorker.ipdl b/dom/serviceworkers/PServiceWorker.ipdl
index 05e8c95c54fa..7ebcbe2225e2 100644
--- a/dom/serviceworkers/PServiceWorker.ipdl
+++ b/dom/serviceworkers/PServiceWorker.ipdl
@@ -4,6 +4,9 @@
 
 include protocol PBackground;
 
+include ClientIPCTypes;
+include DOMTypes;
+
 namespace mozilla {
 namespace dom {
 
@@ -14,6 +17,8 @@ protocol PServiceWorker
 parent:
   async Teardown();
 
+  async PostMessage(ClonedMessageData aClonedData, ClientInfoAndState aSource);
+
 child:
   async __delete__();
 };
diff --git a/dom/serviceworkers/RemoteServiceWorkerImpl.cpp b/dom/serviceworkers/RemoteServiceWorkerImpl.cpp
index 4e919cb4c98c..4341b52fed23 100644
--- a/dom/serviceworkers/RemoteServiceWorkerImpl.cpp
+++ b/dom/serviceworkers/RemoteServiceWorkerImpl.cpp
@@ -6,9 +6,12 @@
 
 #include "RemoteServiceWorkerImpl.h"
 
+#include "mozilla/dom/ClientInfo.h"
+#include "mozilla/dom/ClientState.h"
 #include "mozilla/ipc/BackgroundChild.h"
 #include "mozilla/ipc/PBackgroundChild.h"
 #include "ServiceWorkerChild.h"
+#include "ServiceWorkerCloneData.h"
 
 namespace mozilla {
 namespace dom {
@@ -67,7 +70,19 @@ RemoteServiceWorkerImpl::PostMessage(RefPtr<ServiceWorkerCloneData>&& aData,
                                      const ClientInfo& aClientInfo,
                                      const ClientState& aClientState)
 {
-  // TODO
+  NS_ASSERT_OWNINGTHREAD(RemoteServiceWorkerImpl);
+  if (!mActor) {
+    return;
+  }
+
+  ClonedMessageData data;
+  if (!aData->BuildClonedMessageDataForBackgroundChild(mActor->Manager(),
+                                                       data)) {
+    return;
+  }
+
+  mActor->SendPostMessage(data, ClientInfoAndState(aClientInfo.ToIPC(),
+                                                   aClientState.ToIPC()));
 }
 
 RemoteServiceWorkerImpl::RemoteServiceWorkerImpl(const ServiceWorkerDescriptor& aDescriptor)
diff --git a/dom/serviceworkers/ServiceWorkerParent.cpp b/dom/serviceworkers/ServiceWorkerParent.cpp
index 4aa2f55d4f51..8fb180604645 100644
--- a/dom/serviceworkers/ServiceWorkerParent.cpp
+++ b/dom/serviceworkers/ServiceWorkerParent.cpp
@@ -6,11 +6,16 @@
 
 #include "ServiceWorkerParent.h"
 
+#include "ServiceWorkerCloneData.h"
 #include "ServiceWorkerProxy.h"
+#include "mozilla/dom/ClientInfo.h"
+#include "mozilla/dom/ClientState.h"
+#include "mozilla/dom/ipc/StructuredCloneData.h"
 
 namespace mozilla {
 namespace dom {
 
+using mozilla::dom::ipc::StructuredCloneData;
 using mozilla::ipc::IPCResult;
 
 void
@@ -29,6 +34,19 @@ ServiceWorkerParent::RecvTeardown()
   return IPC_OK();
 }
 
+IPCResult
+ServiceWorkerParent::RecvPostMessage(const ClonedMessageData& aClonedData,
+                                     const ClientInfoAndState& aSource)
+{
+  RefPtr<ServiceWorkerCloneData> data = new ServiceWorkerCloneData();
+  data->CopyFromClonedMessageDataForBackgroundParent(aClonedData);
+
+  mProxy->PostMessage(std::move(data), ClientInfo(aSource.info()),
+                      ClientState::FromIPC(aSource.state()));
+
+  return IPC_OK();
+}
+
 ServiceWorkerParent::ServiceWorkerParent()
   : mDeleteSent(false)
 {
diff --git a/dom/serviceworkers/ServiceWorkerParent.h b/dom/serviceworkers/ServiceWorkerParent.h
index 8cbcf63d358e..d112b689a221 100644
--- a/dom/serviceworkers/ServiceWorkerParent.h
+++ b/dom/serviceworkers/ServiceWorkerParent.h
@@ -27,6 +27,10 @@ class ServiceWorkerParent final : public PServiceWorkerParent
   mozilla::ipc::IPCResult
   RecvTeardown() override;
 
+  mozilla::ipc::IPCResult
+  RecvPostMessage(const ClonedMessageData& aClonedData,
+                  const ClientInfoAndState& aSource) override;
+
 public:
   ServiceWorkerParent();
   ~ServiceWorkerParent();
diff --git a/dom/serviceworkers/ServiceWorkerProxy.cpp b/dom/serviceworkers/ServiceWorkerProxy.cpp
index 64b195ab99f5..dac23d7fcef4 100644
--- a/dom/serviceworkers/ServiceWorkerProxy.cpp
+++ b/dom/serviceworkers/ServiceWorkerProxy.cpp
@@ -115,5 +115,22 @@ ServiceWorkerProxy::RevokeActor(ServiceWorkerParent* aActor)
   MOZ_ALWAYS_SUCCEEDS(SystemGroup::Dispatch(TaskCategory::Other, r.forget()));
 }
 
+void
+ServiceWorkerProxy::PostMessage(RefPtr<ServiceWorkerCloneData>&& aData,
+                                const ClientInfo& aClientInfo,
+                                const ClientState& aClientState)
+{
+  AssertIsOnBackgroundThread();
+  RefPtr<ServiceWorkerProxy> self = this;
+  nsCOMPtr<nsIRunnable> r = NS_NewRunnableFunction(__func__,
+    [self, data = std::move(aData), aClientInfo, aClientState] () mutable {
+      if (!self->mInfo) {
+        return;
+      }
+      self->mInfo->PostMessage(std::move(data), aClientInfo, aClientState);
+    });
+  MOZ_ALWAYS_SUCCEEDS(SystemGroup::Dispatch(TaskCategory::Other, r.forget()));
+}
+
 } // namespace dom
 } // namespace mozilla
diff --git a/dom/serviceworkers/ServiceWorkerProxy.h b/dom/serviceworkers/ServiceWorkerProxy.h
index 6aee20ede3e7..80e2e5554097 100644
--- a/dom/serviceworkers/ServiceWorkerProxy.h
+++ b/dom/serviceworkers/ServiceWorkerProxy.h
@@ -13,6 +13,7 @@
 namespace mozilla {
 namespace dom {
 
+class ServiceWorkerCloneData;
 class ServiceWorkerInfo;
 class ServiceWorkerParent;
 
@@ -56,6 +57,11 @@ public:
   void
   RevokeActor(ServiceWorkerParent* aActor);
 
+  void
+  PostMessage(RefPtr<ServiceWorkerCloneData>&& aData,
+              const ClientInfo& aClientInfo,
+              const ClientState& aState);
+
   NS_INLINE_DECL_THREADSAFE_REFCOUNTING(ServiceWorkerProxy);
 };