diff --git a/autotests/test_window_paint_data.cpp b/autotests/test_window_paint_data.cpp
--- a/autotests/test_window_paint_data.cpp
+++ b/autotests/test_window_paint_data.cpp
@@ -254,6 +254,9 @@
     bool isOutline() const override {
         return false;
     }
+    pid_t pid() const override {
+        return 0;
+    }
 
 private:
     qreal m_opacity = 1.0;
diff --git a/effects.h b/effects.h
--- a/effects.h
+++ b/effects.h
@@ -462,6 +462,8 @@
     bool isWaylandClient() const override;
     bool isX11Client() const override;
 
+    pid_t pid() const override;
+
     QRect decorationInnerRect() const override;
     QByteArray readProperty(long atom, long type, int format) const override;
     void deleteProperty(long atom) const override;
diff --git a/effects.cpp b/effects.cpp
--- a/effects.cpp
+++ b/effects.cpp
@@ -2127,6 +2127,11 @@
     return x11Client;
 }
 
+pid_t EffectWindowImpl::pid() const
+{
+    return toplevel->pid();
+}
+
 
 //****************************************
 // EffectWindowGroupImpl
diff --git a/libkwineffects/kwineffects.h b/libkwineffects/kwineffects.h
--- a/libkwineffects/kwineffects.h
+++ b/libkwineffects/kwineffects.h
@@ -2385,6 +2385,11 @@
      */
     virtual bool isOutline() const = 0;
 
+    /**
+     * @since 5.18
+     */
+    virtual pid_t pid() const = 0;
+
     /**
      * Can be used to by effects to store arbitrary data in the EffectWindow.
      *