diff --git a/lib/libc/amd64/string/Makefile.inc b/lib/libc/amd64/string/Makefile.inc
index 5395ed30772a..f5d69d60fbc5 100644
--- a/lib/libc/amd64/string/Makefile.inc
+++ b/lib/libc/amd64/string/Makefile.inc
@@ -1,4 +1,4 @@
 # $FreeBSD$
 
 MDSRCS+= bcmp.S bcopy.S bzero.S memcmp.S memcpy.S memmove.S memset.S \
-	strcmp.S strcpy.S
+	strcat.S strcmp.S strcpy.S
diff --git a/lib/libc/amd64/string/strcat.S b/lib/libc/amd64/string/strcat.S
new file mode 100644
index 000000000000..78a1b5645eef
--- /dev/null
+++ b/lib/libc/amd64/string/strcat.S
@@ -0,0 +1,165 @@
+/*
+ * Written by J.T. Conklin <jtc@acorntoolworks.com>
+ * Public domain.
+ */
+
+#include <machine/asm.h>
+__FBSDID("$FreeBSD$");
+
+#if 0
+	RCSID("$NetBSD: strcat.S,v 1.4 2004/07/26 18:51:21 drochner Exp $")
+#endif
+
+ENTRY(strcat)
+	movq	%rdi,%rax
+	movabsq	$0x0101010101010101,%r8
+	movabsq	$0x8080808080808080,%r9
+
+	/*
+	 * Align destination to word boundary.
+	 * Consider unrolling loop?
+	 */
+.Lscan:
+.Lscan_align:
+	testb	$7,%dil
+	je	.Lscan_aligned
+	cmpb	$0,(%rdi)
+	je	.Lcopy
+	incq	%rdi
+	jmp	.Lscan_align
+
+	.align	4
+.Lscan_aligned:
+.Lscan_loop:
+	movq	(%rdi),%rdx
+	addq	$8,%rdi
+	subq	%r8,%rdx
+	testq	%r9,%rdx
+	je	.Lscan_loop
+
+	/*
+	 * In rare cases, the above loop may exit prematurely. We must
+	 * return to the loop if none of the bytes in the word equal 0.
+	 */
+
+	cmpb	$0,-8(%rdi)	/* 1st byte == 0? */
+	jne	1f
+	subq	$8,%rdi
+	jmp	.Lcopy
+
+1:	cmpb	$0,-7(%rdi)	/* 2nd byte == 0? */
+	jne	1f
+	subq	$7,%rdi
+	jmp	.Lcopy
+
+1:	cmpb	$0,-6(%rdi)	/* 3rd byte == 0? */
+	jne	1f
+	subq	$6,%rdi
+	jmp	.Lcopy
+
+1:	cmpb	$0,-5(%rdi)	/* 4th byte == 0? */
+	jne	1f
+	subq	$5,%rdi
+	jmp	.Lcopy
+
+1:	cmpb	$0,-4(%rdi)	/* 5th byte == 0? */
+	jne	1f
+	subq	$4,%rdi
+	jmp	.Lcopy
+
+1:	cmpb	$0,-3(%rdi)	/* 6th byte == 0? */
+	jne	1f
+	subq	$3,%rdi
+	jmp	.Lcopy
+
+1:	cmpb	$0,-2(%rdi)	/* 7th byte == 0? */
+	jne	1f
+	subq	$2,%rdi
+	jmp	.Lcopy
+
+1:	cmpb	$0,-1(%rdi)	/* 8th byte == 0? */
+	jne	.Lscan_loop
+	subq	$1,%rdi
+
+	/*
+	 * Align source to a word boundary.
+	 * Consider unrolling loop?
+	 */
+.Lcopy:
+.Lcopy_align:
+	testb	$7,%sil
+	je	.Lcopy_aligned
+	movb	(%rsi),%dl
+	incq	%rsi
+	movb	%dl,(%rdi)
+	incq	%rdi
+	testb	%dl,%dl
+	jne	.Lcopy_align
+	ret
+
+	.align	4
+.Lcopy_loop:
+	movq	%rdx,(%rdi)
+	addq	$8,%rdi
+.Lcopy_aligned:
+	movq	(%rsi),%rdx
+	movq	%rdx,%rcx
+	addq	$8,%rsi
+	subq	%r8,%rcx
+	testq	%r9,%rcx
+	je	.Lcopy_loop
+
+	/*
+	 * In rare cases, the above loop may exit prematurely. We must
+	 * return to the loop if none of the bytes in the word equal 0.
+	 */
+
+	movb	%dl,(%rdi)
+	incq	%rdi
+	testb	%dl,%dl		/* 1st byte == 0? */
+	je	.Ldone
+
+	shrq	$8,%rdx
+	movb	%dl,(%rdi)
+	incq	%rdi
+	testb	%dl,%dl		/* 2nd byte == 0? */
+	je	.Ldone
+
+	shrq	$8,%rdx
+	movb	%dl,(%rdi)
+	incq	%rdi
+	testb	%dl,%dl		/* 3rd byte == 0? */
+	je	.Ldone
+
+	shrq	$8,%rdx
+	movb	%dl,(%rdi)
+	incq	%rdi
+	testb	%dl,%dl		/* 4th byte == 0? */
+	je	.Ldone
+
+	shrq	$8,%rdx
+	movb	%dl,(%rdi)
+	incq	%rdi
+	testb	%dl,%dl		/* 5th byte == 0? */
+	je	.Ldone
+
+	shrq	$8,%rdx
+	movb	%dl,(%rdi)
+	incq	%rdi
+	testb	%dl,%dl		/* 6th byte == 0? */
+	je	.Ldone
+
+	shrq	$8,%rdx
+	movb	%dl,(%rdi)
+	incq	%rdi
+	testb	%dl,%dl		/* 7th byte == 0? */
+	je	.Ldone
+
+	shrq	$8,%rdx
+	movb	%dl,(%rdi)
+	incq	%rdi
+	testb	%dl,%dl		/* 8th byte == 0? */
+	jne	.Lcopy_aligned
+
+.Ldone:
+	ret