diff --git a/modules/rostests/apitests/user32/CMakeLists.txt b/modules/rostests/apitests/user32/CMakeLists.txt
index 896023291c8..f0dca068549 100644
--- a/modules/rostests/apitests/user32/CMakeLists.txt
+++ b/modules/rostests/apitests/user32/CMakeLists.txt
@@ -42,6 +42,7 @@ list(APPEND SOURCE
     SetProp.c
     SetScrollInfo.c
     SetScrollRange.c
+    ShowWindow.c
     SwitchToThisWindow.c
     SystemParametersInfo.c
     TrackMouseEvent.c
diff --git a/modules/rostests/apitests/user32/ShowWindow.c b/modules/rostests/apitests/user32/ShowWindow.c
new file mode 100644
index 00000000000..832688b938e
--- /dev/null
+++ b/modules/rostests/apitests/user32/ShowWindow.c
@@ -0,0 +1,183 @@
+/*
+ * PROJECT:     ReactOS API tests
+ * LICENSE:     LGPL-2.1+ (https://spdx.org/licenses/LGPL-2.1+)
+ * PURPOSE:     Tests for ShowWindow
+ * COPYRIGHT:   Copyright 2021 Katayama Hirofumi MZ <katayama.hirofumi.mz@gmail.com>
+ */
+
+#include "precomp.h"
+
+typedef struct TEST_ENTRY
+{
+    INT lineno;
+    BOOL ret;
+    INT nCmdShow;
+    DWORD style0;
+    DWORD style1;
+} TEST_ENTRY;
+
+static const CHAR s_name[] = "ShowWindow test window";
+
+static void DoTestEntry(const TEST_ENTRY *pEntry)
+{
+    HWND hwnd;
+    DWORD style;
+    BOOL ret;
+
+    hwnd = CreateWindowA(s_name, s_name, pEntry->style0,
+        CW_USEDEFAULT, CW_USEDEFAULT, 100, 100,
+        NULL, NULL, GetModuleHandleA(NULL), NULL);
+    ok(!!hwnd, "Line %d: CreateWindowA failed\n", pEntry->lineno);
+
+    ret = !!ShowWindow(hwnd, pEntry->nCmdShow);
+    ok(ret == pEntry->ret, "Line %d: ShowWindow returned %s\n", pEntry->lineno,
+       (ret ? "non-zero" : "zero"));
+
+    style = (LONG)GetWindowLongPtrA(hwnd, GWL_STYLE);
+    ok(style == pEntry->style1, "Line %d: style was 0x%lX\n", pEntry->lineno, style);
+
+    DestroyWindow(hwnd);
+}
+
+#define STYLE_0 WS_OVERLAPPEDWINDOW
+#define STYLE_1 (WS_OVERLAPPEDWINDOW | WS_MAXIMIZE)
+#define STYLE_2 (WS_OVERLAPPEDWINDOW | WS_MINIMIZE)
+#define STYLE_3 (WS_OVERLAPPEDWINDOW | WS_VISIBLE)
+#define STYLE_4 (WS_OVERLAPPEDWINDOW | WS_VISIBLE | WS_MAXIMIZE)
+#define STYLE_5 (WS_OVERLAPPEDWINDOW | WS_VISIBLE | WS_MINIMIZE)
+
+#define SW_0 -1
+#define SW_1 SW_HIDE
+#define SW_2 SW_MAXIMIZE
+#define SW_3 SW_MINIMIZE
+#define SW_4 SW_RESTORE
+#define SW_5 SW_SHOW
+#define SW_6 SW_SHOWDEFAULT
+#define SW_7 SW_SHOWMAXIMIZED
+#define SW_8 SW_SHOWMINIMIZED
+#define SW_9 SW_SHOWMINNOACTIVE
+#define SW_10 SW_SHOWNA
+#define SW_11 SW_SHOWNOACTIVATE
+#define SW_12 SW_SHOWNORMAL
+
+static const TEST_ENTRY s_entries[] =
+{
+    // STYLE_0
+    { __LINE__, FALSE, SW_0, STYLE_0, STYLE_0 | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_1, STYLE_0, STYLE_0 | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_2, STYLE_0, STYLE_0 | WS_VISIBLE | WS_MAXIMIZE | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_3, STYLE_0, STYLE_0 | WS_VISIBLE | WS_MINIMIZE | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_4, STYLE_0, STYLE_0 | WS_VISIBLE | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_5, STYLE_0, STYLE_0 | WS_VISIBLE | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_6, STYLE_0, STYLE_0 | WS_VISIBLE | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_7, STYLE_0, STYLE_0 | WS_VISIBLE | WS_MAXIMIZE | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_8, STYLE_0, STYLE_0 | WS_VISIBLE | WS_MINIMIZE | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_9, STYLE_0, STYLE_0 | WS_VISIBLE | WS_MINIMIZE | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_10, STYLE_0, STYLE_0 | WS_VISIBLE| WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_11, STYLE_0, STYLE_0 | WS_VISIBLE | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_12, STYLE_0, STYLE_0 | WS_VISIBLE | WS_CLIPSIBLINGS },
+    // STYLE_1
+    { __LINE__, FALSE, SW_0, STYLE_1, STYLE_1 | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_1, STYLE_1, STYLE_1 | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_2, STYLE_1, STYLE_1 | WS_VISIBLE | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_3, STYLE_1, STYLE_3 | WS_MINIMIZE | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_4, STYLE_1, STYLE_3 | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_5, STYLE_1, STYLE_1 | WS_VISIBLE | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_6, STYLE_1, STYLE_3 | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_7, STYLE_1, STYLE_1 | WS_VISIBLE | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_8, STYLE_1, STYLE_3 | WS_MINIMIZE | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_9, STYLE_1, STYLE_3 | WS_MINIMIZE | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_10, STYLE_1, STYLE_1 | WS_VISIBLE | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_11, STYLE_1, STYLE_3 | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_12, STYLE_1, STYLE_3 | WS_CLIPSIBLINGS },
+    // STYLE_2
+    { __LINE__, FALSE, SW_0, STYLE_2, STYLE_2 | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_1, STYLE_2, STYLE_2 | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_2, STYLE_2, STYLE_3 | WS_MAXIMIZE | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_3, STYLE_2, STYLE_2 | WS_VISIBLE | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_4, STYLE_2, STYLE_3 | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_5, STYLE_2, STYLE_3 | WS_MINIMIZE | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_6, STYLE_2, STYLE_3 | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_7, STYLE_2, STYLE_3 | WS_MAXIMIZE | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_8, STYLE_2, STYLE_2 | WS_VISIBLE | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_9, STYLE_2, STYLE_2 | WS_VISIBLE | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_10, STYLE_2, STYLE_3 | WS_MINIMIZE | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_11, STYLE_2, STYLE_3 | WS_CLIPSIBLINGS },
+    { __LINE__, FALSE, SW_12, STYLE_2, STYLE_3 | WS_CLIPSIBLINGS },
+    // STYLE_3
+    { __LINE__, FALSE, SW_0, STYLE_3, STYLE_3 | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_1, STYLE_3, WS_OVERLAPPEDWINDOW | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_2, STYLE_3, STYLE_3 | WS_MAXIMIZE | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_3, STYLE_3, STYLE_3 | WS_MINIMIZE | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_4, STYLE_3, STYLE_3 | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_5, STYLE_3, STYLE_3 | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_6, STYLE_3, STYLE_3 | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_7, STYLE_3, STYLE_3 | WS_MAXIMIZE | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_8, STYLE_3, STYLE_3 | WS_MINIMIZE | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_9, STYLE_3, STYLE_3 | WS_MINIMIZE | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_10, STYLE_3, STYLE_3 | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_11, STYLE_3, STYLE_3 | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_12, STYLE_3, STYLE_3 | WS_CLIPSIBLINGS },
+    // STYLE_4
+    { __LINE__, FALSE, SW_0, STYLE_4, STYLE_4 | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_1, STYLE_4, WS_OVERLAPPEDWINDOW | WS_MAXIMIZE | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_2, STYLE_4, STYLE_4 | WS_MAXIMIZE | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_3, STYLE_4, STYLE_3 | WS_MINIMIZE | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_4, STYLE_4, STYLE_3 | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_5, STYLE_4, STYLE_4 | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_6, STYLE_4, STYLE_3 | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_7, STYLE_4, STYLE_4 | WS_MAXIMIZE | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_8, STYLE_4, STYLE_3 | WS_MINIMIZE | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_9, STYLE_4, STYLE_3 | WS_MINIMIZE | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_10, STYLE_4, STYLE_4 | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_11, STYLE_4, STYLE_3 | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_12, STYLE_4, STYLE_3 | WS_CLIPSIBLINGS },
+    // STYLE_5
+    { __LINE__, FALSE, SW_0, STYLE_5, STYLE_5 | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_1, STYLE_5, WS_OVERLAPPEDWINDOW | WS_MINIMIZE | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_2, STYLE_5, STYLE_3 | WS_MAXIMIZE | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_3, STYLE_5, STYLE_5 | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_4, STYLE_5, STYLE_3 | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_5, STYLE_5, STYLE_5 | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_6, STYLE_5, STYLE_3 | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_7, STYLE_5, STYLE_3 | WS_MAXIMIZE | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_8, STYLE_5, STYLE_5 | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_9, STYLE_5, STYLE_5 | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_10, STYLE_5, STYLE_5 | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_11, STYLE_5, STYLE_3 | WS_CLIPSIBLINGS },
+    { __LINE__, TRUE, SW_12, STYLE_5, STYLE_3 | WS_CLIPSIBLINGS },
+};
+
+static LRESULT CALLBACK
+WindowProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
+{
+    switch (uMsg)
+    {
+        case WM_CREATE:
+            return 0;
+        case WM_DESTROY:
+            PostQuitMessage(0);
+            break;
+        default:
+            return DefWindowProcA(hwnd, uMsg, wParam, lParam);
+    }
+    return 0;
+}
+
+START_TEST(ShowWindow)
+{
+    WNDCLASSA wc;
+    UINT iTest;
+
+    ZeroMemory(&wc, sizeof(wc));
+    wc.lpfnWndProc = WindowProc;
+    wc.hInstance = GetModuleHandleA(NULL);
+    wc.hbrBackground = (HBRUSH)(COLOR_3DFACE + 1);
+    wc.lpszClassName = s_name;
+    ok_int(1, !!RegisterClassA(&wc));
+
+    for (iTest = 0; iTest < _countof(s_entries); ++iTest)
+    {
+        DoTestEntry(&s_entries[iTest]);
+    }
+}
diff --git a/modules/rostests/apitests/user32/testlist.c b/modules/rostests/apitests/user32/testlist.c
index d3b1dda9960..b4ef3e04e4c 100644
--- a/modules/rostests/apitests/user32/testlist.c
+++ b/modules/rostests/apitests/user32/testlist.c
@@ -44,6 +44,7 @@ extern void func_SetParent(void);
 extern void func_SetProp(void);
 extern void func_SetScrollInfo(void);
 extern void func_SetScrollRange(void);
+extern void func_ShowWindow(void);
 extern void func_SwitchToThisWindow(void);
 extern void func_SystemParametersInfo(void);
 extern void func_TrackMouseEvent(void);
@@ -93,6 +94,7 @@ const struct test winetest_testlist[] =
     { "SetProp", func_SetProp },
     { "SetScrollInfo", func_SetScrollInfo },
     { "SetScrollRange", func_SetScrollRange },
+    { "ShowWindow", func_ShowWindow },
     { "SwitchToThisWindow", func_SwitchToThisWindow },
     { "SystemParametersInfo", func_SystemParametersInfo },
     { "TrackMouseEvent", func_TrackMouseEvent },