diff --git a/modules/rostests/win32/user32/CMakeLists.txt b/modules/rostests/win32/user32/CMakeLists.txt
index 44e44fb78a9..c60aa244a7e 100644
--- a/modules/rostests/win32/user32/CMakeLists.txt
+++ b/modules/rostests/win32/user32/CMakeLists.txt
@@ -2,5 +2,6 @@ add_subdirectory(biditext)
 add_subdirectory(messagebox)
 add_subdirectory(paintdesktop)
 add_subdirectory(psmtest)
+add_subdirectory(serverside)
 add_subdirectory(sysicon)
 add_subdirectory(winstation)
diff --git a/modules/rostests/win32/user32/serverside/CMakeLists.txt b/modules/rostests/win32/user32/serverside/CMakeLists.txt
new file mode 100644
index 00000000000..31817f32580
--- /dev/null
+++ b/modules/rostests/win32/user32/serverside/CMakeLists.txt
@@ -0,0 +1,9 @@
+
+list(APPEND SOURCE
+    serverside.c)
+
+add_executable(serverside ${SOURCE})
+target_link_libraries(serverside ${PSEH_LIB})
+set_module_type(serverside win32gui UNICODE)
+add_importlibs(serverside gdi32 user32 msvcrt kernel32 ntdll)
+add_rostests_file(TARGET serverside SUBDIR suppl)
diff --git a/modules/rostests/win32/user32/serverside/serverside.c b/modules/rostests/win32/user32/serverside/serverside.c
new file mode 100644
index 00000000000..54081881a14
--- /dev/null
+++ b/modules/rostests/win32/user32/serverside/serverside.c
@@ -0,0 +1,107 @@
+/*
+ * PROJECT:         ReactOS test suite
+ * LICENSE:         GPL-2.0-or-later (https://spdx.org/licenses/GPL-2.0-or-later)
+ * PURPOSE:         Test for existense of the kernel mode wndproc in the window
+ * COPYRIGHT:       Copyright 2024 Oleg Dubinskiy (oleg.dubinskiy@reactos.org)
+ */
+
+/* This testapp tests behaviour of IsServerSideWindow() function in user32. */
+
+#define _WINE
+
+#include <windows.h>
+#include <stdio.h>
+
+#include <debug.h>
+
+WCHAR WndClass[] = L"window class";
+
+LRESULT CALLBACK WndProc(HWND hWnd,
+                         UINT msg,
+                         WPARAM wParam,
+                         LPARAM lParam)
+{
+    switch (msg)
+    {
+        case WM_PAINT:
+            return 0;
+
+        case WM_DESTROY:
+            PostQuitMessage(0);
+            return 0;
+    }
+
+    return DefWindowProcW(hWnd, msg, wParam, lParam);
+}
+
+int APIENTRY wWinMain(HINSTANCE hInst,
+                      HINSTANCE hPrevInstance,
+                      LPWSTR lpCmdLine,
+                      int nCmdShow)
+{
+    HWND hWnd1a, hWnd1b;
+    WNDCLASSEXW wcx;
+    UINT result;
+    BOOL ret;
+
+    memset(&wcx, 0, sizeof(wcx));
+    wcx.cbSize = sizeof(wcx);
+    wcx.lpfnWndProc = (WNDPROC)WndProc;
+    wcx.hInstance = hInst;
+    wcx.hbrBackground = (HBRUSH)(COLOR_WINDOW + 1);
+    wcx.lpszClassName = WndClass;
+
+    if (!(result = RegisterClassExW(&wcx)))
+        return 1;
+
+    /* 1. Window with a valid wndproc */
+    hWnd1a = CreateWindowExW(0,
+                             WndClass,
+                             NULL,
+                             WS_CAPTION | WS_SYSMENU,
+                             CW_USEDEFAULT, CW_USEDEFAULT,
+                             400, 100,
+                             NULL, 0,
+                             hInst, NULL);
+    if (!hWnd1a)
+        return 1;
+
+    ShowWindow(hWnd1a, SW_SHOW);
+    UpdateWindow(hWnd1a);
+
+    ret = IsServerSideWindow(hWnd1a);
+    if (ret)
+        DPRINT("OK: the window %p has a valid kernel mode wndproc\n", hWnd1a);
+    else
+        DPRINT("FAIL: the window %p is not valid or has no valid kernel mode wndproc\n", hWnd1a);
+
+    // TODO: this seems to be not a correct test condition.
+    //       Find a valid condition to test a kernel mode wmdproc existence correctly!
+
+    //wcx.lpfnWndProc = NULL;
+
+    /* 2. Window without a valid wndproc */
+    hWnd1b = CreateWindowExW(0,
+                             WndClass,
+                             NULL,
+                             WS_CAPTION | WS_SYSMENU,
+                             CW_USEDEFAULT, CW_USEDEFAULT,
+                             400, 100,
+                             NULL, 0,
+                             hInst, NULL);
+
+    if (!hWnd1b)
+        return 1;
+
+    ShowWindow(hWnd1b, SW_SHOW);
+    UpdateWindow(hWnd1b);
+
+    ret = IsServerSideWindow(hWnd1b);
+    if (ret)
+        DPRINT("FAIL: the window %p has a valid kernel mode wndproc when it shouldn't\n", hWnd1b);
+    else
+        DPRINT("OK: the window %p has no valid kernel mode wndproc\n", hWnd1b);
+
+    UnregisterClassW(WndClass, hInst);
+    return 0;
+}
diff --git a/sdk/include/reactos/undocuser.h b/sdk/include/reactos/undocuser.h
index ab289d056b9..f7cfa4350ef 100644
--- a/sdk/include/reactos/undocuser.h
+++ b/sdk/include/reactos/undocuser.h
@@ -199,6 +199,7 @@ LONG WINAPI CsrBroadcastSystemMessageExW(DWORD dwflags,
                                          LPARAM lParam,
                                          PBSMINFO pBSMInfo);
 BOOL WINAPI CliImmSetHotKey(DWORD dwID, UINT uModifiers, UINT uVirtualKey, HKL hKl);
+BOOL WINAPI IsServerSideWindow(HWND);
 HWND WINAPI SetTaskmanWindow(HWND);
 HWND WINAPI GetTaskmanWindow(VOID);
 HWND WINAPI GetProgmanWindow(VOID);
diff --git a/win32ss/user/user32/misc/stubs.c b/win32ss/user/user32/misc/stubs.c
index bb1754b781f..0936064e19c 100644
--- a/win32ss/user/user32/misc/stubs.c
+++ b/win32ss/user/user32/misc/stubs.c
@@ -432,15 +432,6 @@ WORD WINAPI InitializeWin32EntryTable(UCHAR* EntryTablePlus0x1000)
   return FALSE;
 }
 
-/*
- * @unimplemented
- */
-BOOL WINAPI IsServerSideWindow(HWND wnd)
-{
-  UNIMPLEMENTED;
-  return FALSE;
-}
-
 /*
  * @unimplemented
  */
diff --git a/win32ss/user/user32/windows/window.c b/win32ss/user/user32/windows/window.c
index e7468f8a91c..a0fb03e41dd 100644
--- a/win32ss/user/user32/windows/window.c
+++ b/win32ss/user/user32/windows/window.c
@@ -1522,6 +1522,37 @@ IsIconic(HWND hWnd)
 }
 
 
+/**
+ * @brief
+ * IsServerSideWindow
+ *
+ * Checks whether a window has a window procedure that resides in kernel mode.
+ *
+ * @param[in] hWnd
+ * A handle to the window to be checked.
+ *
+ * @return
+ * TRUE if the window has a window procedure that resides in kernel mode,
+ * FALSE otherwise.
+ *
+ * @remarks
+ * Undocumented, see http://undoc.airesoft.co.uk/user32.dll/IsServerSideWindow.php
+ * (unofficial documentation).
+ */
+BOOL
+WINAPI
+IsServerSideWindow(
+    _In_ HWND hWnd)
+{
+    PWND Wnd = ValidateHwnd(hWnd);
+
+    if (Wnd != NULL)
+        return (Wnd->state & WNDS_SERVERSIDEWINDOWPROC) != 0;
+
+    return FALSE;
+}
+
+
 /*
  * @implemented
  */