diff --git a/Libraries/LibGUI/GWindow.cpp b/Libraries/LibGUI/GWindow.cpp
index d341bef2564..997b587e789 100644
--- a/Libraries/LibGUI/GWindow.cpp
+++ b/Libraries/LibGUI/GWindow.cpp
@@ -714,3 +714,18 @@ void GWindow::save_to(AK::JsonObject& json)
     json.set("size_increment", size_increment().to_string());
     CObject::save_to(json);
 }
+
+void GWindow::set_fullscreen(bool fullscreen)
+{
+    if (m_fullscreen == fullscreen)
+        return;
+    m_fullscreen = fullscreen;
+    if (!m_window_id)
+        return;
+
+    WSAPI_ClientMessage request;
+    request.type = WSAPI_ClientMessage::Type::SetFullscreen;
+    request.window_id = m_window_id;
+    request.value = fullscreen;
+    GWindowServerConnection::the().sync_request(request, WSAPI_ServerMessage::Type::DidSetFullscreen);
+}
diff --git a/Libraries/LibGUI/GWindow.h b/Libraries/LibGUI/GWindow.h
index e563cb13229..7591a66ad8a 100644
--- a/Libraries/LibGUI/GWindow.h
+++ b/Libraries/LibGUI/GWindow.h
@@ -33,7 +33,7 @@ public:
     void set_modal(bool);
 
     bool is_fullscreen() const { return m_fullscreen; }
-    void set_fullscreen(bool fullscreen) { m_fullscreen = fullscreen; }
+    void set_fullscreen(bool);
 
     bool is_resizable() const { return m_resizable; }
     void set_resizable(bool resizable) { m_resizable = resizable; }
diff --git a/Servers/WindowServer/WSAPITypes.h b/Servers/WindowServer/WSAPITypes.h
index 501ba20ba0a..bdb14ebf84c 100644
--- a/Servers/WindowServer/WSAPITypes.h
+++ b/Servers/WindowServer/WSAPITypes.h
@@ -113,6 +113,7 @@ struct WSAPI_ServerMessage {
         DidSetWindowHasAlphaChannel,
         ScreenRectChanged,
         ClipboardContentsChanged,
+        DidSetFullscreen,
 
         __Begin_WM_Events__,
         WM_WindowRemoved,
@@ -237,6 +238,7 @@ struct WSAPI_ClientMessage {
         SetWindowHasAlphaChannel,
         MoveWindowToFront,
         SetWindowIconBitmap,
+        SetFullscreen,
     };
     Type type { Invalid };
     int window_id { -1 };
diff --git a/Servers/WindowServer/WSClientConnection.cpp b/Servers/WindowServer/WSClientConnection.cpp
index c583c304997..6bf2dae12fc 100644
--- a/Servers/WindowServer/WSClientConnection.cpp
+++ b/Servers/WindowServer/WSClientConnection.cpp
@@ -330,6 +330,9 @@ bool WSClientConnection::handle_message(const WSAPI_ClientMessage& message, cons
     case WSAPI_ClientMessage::Type::MoveWindowToFront:
         CEventLoop::current().post_event(*this, make<WSAPIMoveWindowToFrontRequest>(client_id(), message.window_id));
         break;
+    case WSAPI_ClientMessage::Type::SetFullscreen:
+        CEventLoop::current().post_event(*this, make<WSAPISetFullscreenRequest>(client_id(), message.window_id, message.value));
+        break;
     default:
         break;
     }
@@ -544,6 +547,23 @@ void WSClientConnection::handle_request(const WSAPIMoveWindowToFrontRequest& req
     WSWindowManager::the().move_to_front_and_make_active(window);
 }
 
+void WSClientConnection::handle_request(const WSAPISetFullscreenRequest& request)
+{
+    int window_id = request.window_id();
+    auto it = m_windows.find(window_id);
+    if (it == m_windows.end()) {
+        post_error("WSAPISetFullscreenRequest: Bad window ID");
+        return;
+    }
+    auto& window = *(*it).value;
+    window.set_fullscreen(request.fullscreen());
+
+    WSAPI_ServerMessage response;
+    response.type = WSAPI_ServerMessage::Type::DidSetFullscreen;
+    response.window_id = window_id;
+    post_message(response);
+}
+
 void WSClientConnection::handle_request(const WSAPISetWindowOpacityRequest& request)
 {
     int window_id = request.window_id();
@@ -1038,6 +1058,8 @@ void WSClientConnection::on_request(const WSAPIClientRequest& request)
         return handle_request(static_cast<const WSAPISetWindowHasAlphaChannelRequest&>(request));
     case WSEvent::APIMoveWindowToFrontRequest:
         return handle_request(static_cast<const WSAPIMoveWindowToFrontRequest&>(request));
+    case WSEvent::APISetFullscreenRequest:
+        return handle_request(static_cast<const WSAPISetFullscreenRequest&>(request));
     default:
         break;
     }
diff --git a/Servers/WindowServer/WSClientConnection.h b/Servers/WindowServer/WSClientConnection.h
index 65fc37de6a7..55cdec0bc32 100644
--- a/Servers/WindowServer/WSClientConnection.h
+++ b/Servers/WindowServer/WSClientConnection.h
@@ -84,6 +84,7 @@ private:
     void handle_request(const WSAPIDismissMenuRequest&);
     void handle_request(const WSAPISetWindowHasAlphaChannelRequest&);
     void handle_request(const WSAPIMoveWindowToFrontRequest&);
+    void handle_request(const WSAPISetFullscreenRequest&);
 
     void post_error(const String&);
 
diff --git a/Servers/WindowServer/WSEvent.h b/Servers/WindowServer/WSEvent.h
index d829b432158..0a6f556edac 100644
--- a/Servers/WindowServer/WSEvent.h
+++ b/Servers/WindowServer/WSEvent.h
@@ -65,6 +65,7 @@ public:
         APISetWindowOverrideCursorRequest,
         APISetWindowHasAlphaChannelRequest,
         APIMoveWindowToFrontRequest,
+        APISetFullscreenRequest,
         WMAPISetActiveWindowRequest,
         WMAPISetWindowMinimizedRequest,
         WMAPIStartWindowResizeRequest,
@@ -502,6 +503,23 @@ private:
     int m_window_id { 0 };
 };
 
+class WSAPISetFullscreenRequest final : public WSAPIClientRequest {
+public:
+    explicit WSAPISetFullscreenRequest(int client_id, int window_id, bool fullscreen)
+        : WSAPIClientRequest(WSEvent::APISetFullscreenRequest, client_id)
+        , m_window_id(window_id)
+        , m_fullscreen(fullscreen)
+    {
+    }
+
+    int window_id() const { return m_window_id; }
+    bool fullscreen() const { return m_fullscreen; }
+
+private:
+    int m_window_id { 0 };
+    bool m_fullscreen;
+};
+
 class WSAPISetClipboardContentsRequest final : public WSAPIClientRequest {
 public:
     explicit WSAPISetClipboardContentsRequest(int client_id, int shared_buffer_id, int size, const String& data_type)
diff --git a/Servers/WindowServer/WSWindow.cpp b/Servers/WindowServer/WSWindow.cpp
index 93b037ce97a..18a235a1d6e 100644
--- a/Servers/WindowServer/WSWindow.cpp
+++ b/Servers/WindowServer/WSWindow.cpp
@@ -1,6 +1,7 @@
 #include "WSWindow.h"
 #include "WSEvent.h"
 #include "WSEventLoop.h"
+#include "WSScreen.h"
 #include "WSWindowManager.h"
 #include <WindowServer/WSAPITypes.h>
 #include <WindowServer/WSClientConnection.h>
@@ -354,3 +355,19 @@ void WSWindow::request_close()
     WSEvent close_request(WSEvent::WindowCloseRequest);
     event(close_request);
 }
+
+void WSWindow::set_fullscreen(bool fullscreen)
+{
+    if (m_fullscreen == fullscreen)
+        return;
+    m_fullscreen = fullscreen;
+    Rect new_window_rect = m_rect;
+    if (m_fullscreen) {
+        m_saved_nonfullscreen_rect = m_rect;
+        new_window_rect = WSScreen::the().rect();
+    } else if (!m_saved_nonfullscreen_rect.is_empty()) {
+        new_window_rect = m_saved_nonfullscreen_rect;
+    }
+    CEventLoop::current().post_event(*this, make<WSResizeEvent>(m_rect, new_window_rect));
+    set_rect(new_window_rect);
+}
diff --git a/Servers/WindowServer/WSWindow.h b/Servers/WindowServer/WSWindow.h
index aa01ae77312..c014c9df811 100644
--- a/Servers/WindowServer/WSWindow.h
+++ b/Servers/WindowServer/WSWindow.h
@@ -38,6 +38,7 @@ public:
     void set_maximized(bool);
 
     bool is_fullscreen() const { return m_fullscreen; }
+    void set_fullscreen(bool);
 
     bool show_titlebar() const { return m_show_titlebar; }
     void set_show_titlebar(bool show) { m_show_titlebar = show; }
@@ -155,6 +156,7 @@ private:
     WSClientConnection* m_client { nullptr };
     String m_title;
     Rect m_rect;
+    Rect m_saved_nonfullscreen_rect;
     WSWindowType m_type { WSWindowType::Normal };
     bool m_global_cursor_tracking_enabled { false };
     bool m_automatic_cursor_tracking_enabled { false };