diff --git a/modules/rostests/apitests/user32/CMakeLists.txt b/modules/rostests/apitests/user32/CMakeLists.txt
index 47188cc34d8..da9e7083400 100644
--- a/modules/rostests/apitests/user32/CMakeLists.txt
+++ b/modules/rostests/apitests/user32/CMakeLists.txt
@@ -29,6 +29,7 @@ list(APPEND SOURCE
     GetWindowPlacement.c
     GW_ENABLEDPOPUP.c
     InitializeLpkHooks.c
+    IsServerSideWindow.c
     KbdLayout.c
     keybd_event.c
     LoadImage.c
diff --git a/modules/rostests/apitests/user32/IsServerSideWindow.c b/modules/rostests/apitests/user32/IsServerSideWindow.c
new file mode 100644
index 00000000000..3418c6f54df
--- /dev/null
+++ b/modules/rostests/apitests/user32/IsServerSideWindow.c
@@ -0,0 +1,104 @@
+/*
+ * PROJECT:     ReactOS API tests
+ * LICENSE:     MIT (https://spdx.org/licenses/MIT)
+ * PURPOSE:     Test for IsServerSideWindow
+ * COPYRIGHT:   Copyright 2024 Oleg Dubinskiy <oleg.dubinskiy@reactos.org>
+ *              Copyright 2026 Mohammad Amin Mollazadeh <madamin@pm.me>
+ */
+
+#include "precomp.h"
+
+static const WCHAR WndClass[] = L"window class";
+
+static LRESULT
+CALLBACK
+Test_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);
+}
+
+START_TEST(IsServerSideWindow)
+{
+    HWND hWnd;
+    WNDCLASSEXW wcx;
+    BOOL ret;
+
+    ZeroMemory(&wcx, sizeof(wcx));
+    wcx.cbSize = sizeof(wcx);
+    wcx.lpfnWndProc = Test_WndProc;
+    wcx.hInstance = GetModuleHandleW(NULL);
+    wcx.hbrBackground = (HBRUSH)(COLOR_WINDOW + 1);
+    wcx.lpszClassName = WndClass;
+
+    if (!RegisterClassExW(&wcx))
+    {
+        skip("RegisterClassExW failed with error %lu\n", GetLastError());
+        return;
+    }
+
+    /* 1. Invalid window */
+    hWnd = (HWND)(UINT_PTR)0xdeadbeef;
+    SetLastError(0xfeedfab1);
+    ret = IsServerSideWindow(hWnd);
+    ok(!ret, "The window %p is invalid but IsServerSideWindow() returned TRUE\n", hWnd);
+    ok_eq_ulong(GetLastError(), ERROR_INVALID_WINDOW_HANDLE);
+
+    /* 2. Window with a kernel-mode WndProc.
+     * ScrollBar is an example of a server-side window that can be created from user-mode code. */
+    hWnd = CreateWindowExW(0,
+                           L"ScrollBar",
+                           NULL,
+                           SBS_HORZ | WS_VISIBLE,
+                           CW_USEDEFAULT, CW_USEDEFAULT,
+                           400, 100,
+                           NULL, 0,
+                           wcx.hInstance, NULL);
+    if (!hWnd)
+    {
+        skip("CreateWindowExW failed with error %lu\n", GetLastError());
+        goto Quit;
+    }
+
+    SetLastError(0xfeedfab1);
+    ret = IsServerSideWindow(hWnd);
+    ok(ret, "The window %p is invalid or doesn't have a valid kernel-mode WndProc\n", hWnd);
+    ok_eq_ulong(GetLastError(), 0xfeedfab1); // The last-error shouldn't change.
+
+    DestroyWindow(hWnd);
+
+    /* 3. Window with a user-mode WndProc */
+    hWnd = CreateWindowExW(0,
+                           WndClass,
+                           NULL,
+                           WS_CAPTION | WS_SYSMENU,
+                           CW_USEDEFAULT, CW_USEDEFAULT,
+                           400, 100,
+                           NULL, 0,
+                           wcx.hInstance, NULL);
+    if (!hWnd)
+    {
+        skip("CreateWindowExW failed with error %lu\n", GetLastError());
+        goto Quit;
+    }
+
+    SetLastError(0xfeedfab1);
+    ret = IsServerSideWindow(hWnd);
+    ok(!ret, "The window %p has a valid kernel-mode WndProc when it should not\n", hWnd);
+    ok_eq_ulong(GetLastError(), 0xfeedfab1); // The last-error shouldn't change.
+
+    DestroyWindow(hWnd);
+
+Quit:
+    UnregisterClassW(WndClass, wcx.hInstance);
+    return;
+}
diff --git a/modules/rostests/apitests/user32/testlist.c b/modules/rostests/apitests/user32/testlist.c
index a73593f3153..4bc21ef39fc 100644
--- a/modules/rostests/apitests/user32/testlist.c
+++ b/modules/rostests/apitests/user32/testlist.c
@@ -29,6 +29,7 @@ extern void func_GetUserObjectInformation(void);
 extern void func_GetWindowPlacement(void);
 extern void func_GW_ENABLEDPOPUP(void);
 extern void func_InitializeLpkHooks(void);
+extern void func_IsServerSideWindow(void);
 extern void func_KbdLayout(void);
 extern void func_keybd_event(void);
 extern void func_LoadImage(void);
@@ -94,6 +95,7 @@ const struct test winetest_testlist[] =
     { "GetWindowPlacement", func_GetWindowPlacement },
     { "GW_ENABLEDPOPUP", func_GW_ENABLEDPOPUP },
     { "InitializeLpkHooks", func_InitializeLpkHooks },
+    { "IsServerSideWindow", func_IsServerSideWindow },
     { "KbdLayout", func_KbdLayout },
     { "keybd_event", func_keybd_event },
     { "LoadImage", func_LoadImage },