diff --git a/reactos/dll/win32/msvcrt/msvcrt.spec b/reactos/dll/win32/msvcrt/msvcrt.spec
index db1d101a047..7a20ce62d7c 100644
--- a/reactos/dll/win32/msvcrt/msvcrt.spec
+++ b/reactos/dll/win32/msvcrt/msvcrt.spec
@@ -1026,7 +1026,7 @@
 # stub _wcsicoll_l
 @ cdecl _wcslwr(wstr)
 # stub _wcslwr_l
-# stub _wcslwr_s
+@ cdecl _wcslwr_s(wstr long)
 # stub _wcslwr_s_l
 @ cdecl _wcsncoll(wstr wstr long)
 # stub _wcsncoll_l
diff --git a/reactos/lib/sdk/crt/crt.cmake b/reactos/lib/sdk/crt/crt.cmake
index c01e1756c90..eae70e5d520 100644
--- a/reactos/lib/sdk/crt/crt.cmake
+++ b/reactos/lib/sdk/crt/crt.cmake
@@ -262,6 +262,7 @@ list(APPEND CRT_SOURCE
     string/_mbstrnlen.c
     string/_splitpath.c
     string/_splitpath_s.c
+    string/_wcslwr_s.c
     string/_wsplitpath.c
     string/_wsplitpath_s.c
     string/atof.c
diff --git a/reactos/lib/sdk/crt/string/_wcslwr_s.c b/reactos/lib/sdk/crt/string/_wcslwr_s.c
new file mode 100644
index 00000000000..b760cc9d075
--- /dev/null
+++ b/reactos/lib/sdk/crt/string/_wcslwr_s.c
@@ -0,0 +1,38 @@
+/*
+ * The C RunTime DLL
+ *
+ * Implements C run-time functionality as known from UNIX.
+ *
+ * Copyright 1996,1998 Marcus Meissner
+ * Copyright 1996 Jukka Iivonen
+ * Copyright 1997 Uwe Bonnes
+ */
+
+#include <precomp.h>
+
+/*
+ * @implemented
+ */
+int _wcslwr_s(wchar_t* str, size_t n)
+{
+  wchar_t *ptr=str;
+  if (!str || !n)
+  {
+    if (str) *str = '\0';
+    *_errno() = EINVAL;
+    return EINVAL;
+  }
+
+  while (n--)
+  {
+    if (!*ptr) return 0;
+    *ptr = towlower(*ptr);
+    ptr++;
+  }
+
+  /* MSDN claims that the function should return and set errno to
+   * ERANGE, which doesn't seem to be true based on the tests. */
+  *str = '\0';
+  *_errno() = EINVAL;
+  return EINVAL;
+}