diff --git a/reactos/lib/sdk/crt/string/_mbsnlen.c b/reactos/lib/sdk/crt/string/_mbsnlen.c
new file mode 100644
index 00000000000..180b9b8ca0d
--- /dev/null
+++ b/reactos/lib/sdk/crt/string/_mbsnlen.c
@@ -0,0 +1,36 @@
+/*
+ * COPYRIGHT:         See COPYING in the top level directory
+ * PROJECT:           ReactOS CRT
+ * PURPOSE:           Implementation of _mbsnlen
+ * FILE:              lib/sdk/crt/string/_mbsnlen.c
+ * PROGRAMMER:        Timo Kreuzer
+ */
+
+#include <mbstring.h>
+
+_Check_return_
+_CRTIMP
+size_t
+__cdecl
+_mbsnlen(
+    _In_z_ const unsigned char *pmbstr,
+    _In_ size_t cjMaxLen)
+{
+    size_t cchCount = 0;
+    unsigned char jMbsByte;
+
+    /* Loop while we have bytes to process */
+    while (cjMaxLen-- > 0)
+    {
+        /* Get next mb byte */
+        jMbsByte = *pmbstr++;
+
+        /* If this is 0, we're done */
+        if (jMbsByte == 0) break;
+
+        /* Don't count lead bytes */
+        if (!_ismbblead(jMbsByte)) cchCount++;
+    }
+
+    return cchCount;
+}
diff --git a/reactos/lib/sdk/crt/string/_mbstrnlen.c b/reactos/lib/sdk/crt/string/_mbstrnlen.c
new file mode 100644
index 00000000000..5a856a3cacb
--- /dev/null
+++ b/reactos/lib/sdk/crt/string/_mbstrnlen.c
@@ -0,0 +1,53 @@
+/*
+ * COPYRIGHT:         See COPYING in the top level directory
+ * PROJECT:           ReactOS CRT
+ * PURPOSE:           Implementation of _mbstrnlen
+ * FILE:              lib/sdk/crt/string/_mbstrnlen.c
+ * PROGRAMMER:        Timo Kreuzer
+ */
+
+#include <precomp.h>
+#include <mbctype.h>
+#include <specstrings.h>
+
+_Check_return_
+_CRTIMP
+size_t
+__cdecl
+_mbstrnlen(
+    _In_z_ const char *pmbstr,
+    _In_ size_t cjMaxLen)
+{
+    size_t cchCount = 0;
+    unsigned char jMbsByte;
+
+    /* Check parameters */
+    if (!MSVCRT_CHECK_PMT((pmbstr != 0)) && (cjMaxLen <= INT_MAX))
+    {
+        _set_errno(EINVAL);
+        return -1;
+    }
+
+    /* Loop while we have bytes to process */
+    while (cjMaxLen-- > 0)
+    {
+        /* Get next mb byte */
+        jMbsByte = *pmbstr++;
+
+        /* If this is 0, we're done */
+        if (jMbsByte == 0) break;
+
+        /* if this is a lead byte, continue with next char */
+        if (_ismbblead(jMbsByte))
+        {
+            // FIXME: check if this is a valid char.
+            continue;
+        }
+
+        /* Count this character */
+        cchCount++;
+    }
+
+    return cchCount;
+}
+