diff --git a/dll/ntdll/def/ntdll.spec b/dll/ntdll/def/ntdll.spec
index b7a9ef7851f..cef403e8842 100644
--- a/dll/ntdll/def/ntdll.spec
+++ b/dll/ntdll/def/ntdll.spec
@@ -1888,7 +1888,7 @@
 @ varargs swprintf(ptr wstr)
 @ cdecl -arch=i386,x86_64 tan(double)
 @ cdecl tolower(long)
-@ cdecl toupper(long)
+@ cdecl toupper(long) toupper_nt_mb
 @ cdecl towlower(long)
 @ cdecl towupper(long)
 @ stdcall vDbgPrintEx(long long str ptr)
diff --git a/ntoskrnl/ntoskrnl.spec b/ntoskrnl/ntoskrnl.spec
index db7d01c16a5..300810317d6 100644
--- a/ntoskrnl/ntoskrnl.spec
+++ b/ntoskrnl/ntoskrnl.spec
@@ -1623,7 +1623,7 @@
 @ cdecl strstr()
 @ cdecl swprintf()
 @ cdecl tolower()
-@ cdecl toupper()
+@ cdecl toupper() toupper_nt_mb
 @ cdecl towlower()
 @ cdecl towupper()
 @ stdcall vDbgPrintEx(long long str ptr)
diff --git a/sdk/lib/crt/string/string.cmake b/sdk/lib/crt/string/string.cmake
index 214901c548f..5dd203e2117 100644
--- a/sdk/lib/crt/string/string.cmake
+++ b/sdk/lib/crt/string/string.cmake
@@ -113,6 +113,7 @@ list(APPEND CRT_STRING_ASM_SOURCE
 list(APPEND LIBCNTPR_STRING_SOURCE
     string/mbstowcs_nt.c
     string/tolower_nt.c
+    string/toupper_nt_mb.c
     string/towupper_nt.c
     string/wcstombs_nt.c
 )
diff --git a/sdk/lib/crt/string/toupper_nt_mb.c b/sdk/lib/crt/string/toupper_nt_mb.c
new file mode 100644
index 00000000000..50764523bc6
--- /dev/null
+++ b/sdk/lib/crt/string/toupper_nt_mb.c
@@ -0,0 +1,33 @@
+/*
+ * PROJECT:     ReactOS NT CRT library
+ * LICENSE:     MIT (https://spdx.org/licenses/MIT)
+ * PURPOSE:     Multibyte capable version of toupper
+ * COPYRIGHT:   Copyright 2025 Timo Kreuzer <timo.kreuzer@reactos.org>
+ */
+
+#define WIN32_NO_STATUS
+#include <windef.h>
+#include <ndk/rtlfuncs.h>
+
+_Check_return_
+int
+__cdecl
+toupper_nt_mb(_In_ int _C)
+{
+    PUCHAR ptr;
+    WCHAR wc, wcUpper;
+    CHAR chrUpper[2];
+    ULONG mbSize;
+
+    ptr = (PUCHAR)&_C;
+    wc = RtlAnsiCharToUnicodeChar(&ptr);
+    wcUpper = RtlUpcaseUnicodeChar(wc);
+    RtlUnicodeToMultiByteN(chrUpper, 2, &mbSize, &wcUpper, sizeof(WCHAR));
+
+    if (mbSize == 2)
+        return (UCHAR)chrUpper[1] + ((UCHAR)chrUpper[0] << 8);
+    else if (mbSize == 1)
+        return (UCHAR)chrUpper[0];
+    else
+        return (WCHAR)_C;
+}