diff --git a/lib/libc/amd64/string/Makefile.inc b/lib/libc/amd64/string/Makefile.inc
--- a/lib/libc/amd64/string/Makefile.inc
+++ b/lib/libc/amd64/string/Makefile.inc
@@ -13,6 +13,7 @@
 	strcmp.S \
 	strcpy.c \
 	strcspn.S \
+	strlcat.c \
 	strlcpy.S \
 	strlen.S \
 	strncmp.S \
diff --git a/lib/libc/amd64/string/strlcat.c b/lib/libc/amd64/string/strlcat.c
new file mode 100644
--- /dev/null
+++ b/lib/libc/amd64/string/strlcat.c
@@ -0,0 +1,25 @@
+/*-
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (c) 2023 Robert Clausecker
+ */
+
+#include <sys/cdefs.h>
+
+#include <string.h>
+
+void *__memchr(const void *, int, size_t);
+size_t __strlcpy(char *restrict, const char *restrict, size_t);
+
+size_t
+strlcat(char *restrict dst, const char *restrict src, size_t dstsize)
+{
+	char *loc = __memchr(dst, '\0', dstsize);
+
+	if (loc != NULL) {
+		size_t dstlen = (size_t)(loc - dst);
+
+		return (dstlen + __strlcpy(loc, src, dstsize - dstlen));
+	} else
+		return (dstsize + strlen(src));
+}