diff --git a/autotests/integration/internal_window.cpp b/autotests/integration/internal_window.cpp
--- a/autotests/integration/internal_window.cpp
+++ b/autotests/integration/internal_window.cpp
@@ -57,6 +57,7 @@
     void testKeyboardShowWithoutActivating();
     void testKeyboardTriggersLeave();
     void testTouch();
+    void testOpacity();
 };
 
 class HelperWindow : public QRasterWindow
@@ -494,6 +495,29 @@
     QCOMPARE(win.pressedButtons(), Qt::MouseButtons());
 }
 
+void InternalWindowTest::testOpacity()
+{
+    // this test verifies that opacity is properly synced from QWindow to ShellClient
+    QSignalSpy clientAddedSpy(waylandServer(), &WaylandServer::shellClientAdded);
+    QVERIFY(clientAddedSpy.isValid());
+    HelperWindow win;
+    win.setOpacity(0.5);
+    win.setGeometry(0, 0, 100, 100);
+    win.show();
+    QVERIFY(clientAddedSpy.wait());
+    QCOMPARE(clientAddedSpy.count(), 1);
+    auto internalClient = clientAddedSpy.first().first().value<ShellClient*>();
+    QVERIFY(internalClient);
+    QVERIFY(internalClient->isInternal());
+    QCOMPARE(internalClient->opacity(), 0.5);
+
+    QSignalSpy opacityChangedSpy(internalClient, &ShellClient::opacityChanged);
+    QVERIFY(opacityChangedSpy.isValid());
+    win.setOpacity(0.75);
+    QCOMPARE(opacityChangedSpy.count(), 1);
+    QCOMPARE(internalClient->opacity(), 0.75);
+}
+
 }
 
 WAYLANDTEST_MAIN(KWin::InternalWindowTest)
diff --git a/shell_client.cpp b/shell_client.cpp
--- a/shell_client.cpp
+++ b/shell_client.cpp
@@ -931,13 +931,15 @@
         connect(m_internalWindow, &QWindow::xChanged, this, &ShellClient::updateInternalWindowGeometry);
         connect(m_internalWindow, &QWindow::yChanged, this, &ShellClient::updateInternalWindowGeometry);
         connect(m_internalWindow, &QWindow::destroyed, this, [this] { m_internalWindow = nullptr; });
+        connect(m_internalWindow, &QWindow::opacityChanged, this, &ShellClient::setOpacity);
 
         // Try reading the window type from the QWindow. PlasmaCore.Dialog provides a dynamic type property
         // let's check whether it exists, if it does it's our window type
         const QVariant windowType = m_internalWindow->property("type");
         if (!windowType.isNull()) {
             m_windowType = static_cast<NET::WindowType>(windowType.toInt());
         }
+        setOpacity(m_internalWindow->opacity());
         return;
     }
 }