diff --git a/lib/libc/amd64/string/Makefile.inc b/lib/libc/amd64/string/Makefile.inc
index b569d2cb8be8..a14e8a768f01 100644
--- a/lib/libc/amd64/string/Makefile.inc
+++ b/lib/libc/amd64/string/Makefile.inc
@@ -1,28 +1,29 @@
 MDSRCS+= \
 	amd64_archlevel.c \
 	bcmp.S \
 	memchr.S \
 	memcmp.S \
 	memccpy.S \
 	memcpy.S \
 	memmove.S \
 	memset.S \
 	stpcpy.S \
 	stpncpy.S \
 	strcat.S \
 	strchrnul.S \
 	strcmp.S \
 	strcpy.c \
 	strcspn.S \
 	strlcat.c \
 	strlcpy.S \
 	strlen.S \
+	strncat.c \
 	strncmp.S \
 	strncpy.c \
 	strnlen.c \
 	strpbrk.c \
 	strrchr.S \
 	strsep.c \
 	strspn.S \
 	timingsafe_bcmp.S \
 	timingsafe_memcmp.S
diff --git a/lib/libc/amd64/string/strncat.c b/lib/libc/amd64/string/strncat.c
new file mode 100644
index 000000000000..33b278ac5e04
--- /dev/null
+++ b/lib/libc/amd64/string/strncat.c
@@ -0,0 +1,29 @@
+/*-
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (c) 2023 Robert Clausecker
+ */
+
+#include <sys/cdefs.h>
+
+#include <string.h>
+
+void *__memccpy(void *restrict, const void *restrict, int, size_t);
+
+char *
+strncat(char *dest, const char *src, size_t n)
+{
+	size_t len;
+	char *endptr;
+
+	len = strlen(dest);
+	endptr = __memccpy(dest + len, src, '\0', n);
+
+	/* avoid an extra branch */
+	if (endptr == NULL)
+		endptr = dest + len + n + 1;
+
+	endptr[-1] = '\0';
+
+	return (dest);
+}