diff --git a/lib/libc/aarch64/string/Makefile.inc b/lib/libc/aarch64/string/Makefile.inc
--- a/lib/libc/aarch64/string/Makefile.inc
+++ b/lib/libc/aarch64/string/Makefile.inc
@@ -20,6 +20,8 @@
 	strnlen \
 	strrchr
 
+MDSRCS+=	strspn.S
+
 #
 # Add the above functions. Generate an asm file that includes the needed
 # Arm Optimized Routines file defining the function name to the libc name.
diff --git a/lib/libc/aarch64/string/strspn.S b/lib/libc/aarch64/string/strspn.S
new file mode 100644
--- /dev/null
+++ b/lib/libc/aarch64/string/strspn.S
@@ -0,0 +1,111 @@
+/*-
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (c) 2024 Getz Mikalsen <getz@FreeBSD.org>
+*/
+
+#include <machine/asm.h>
+
+	.weak	strspn
+	.set	strspn, __strspn
+	.text
+
+ENTRY(__strspn)
+
+	/* check for special cases */
+	ldrb	w4, [x1]		// first character in set
+	cbz	w4, .Lzero		// empty set always returns 0
+
+	mov	x15, #1			// preload register with 1 for stores
+
+	// set is only one character
+	ldrb	w5, [x1, #1]		// second character in the set
+	cbz	w5, .Lsingle
+
+	stp	x29, x30, [sp, #-16]!
+	mov	x29, sp
+	sub	sp, sp, #256		// allocate 256 bytes on the stack
+
+	/* no special case matches -- prepare lookup table */
+	mov	w3, #28
+0:	add	x9, sp, x3, lsl #3
+	stp	xzr, xzr, [x9]
+	stp	xzr, xzr, [x9, #16]
+	subs	w3, w3, #4
+	b.cs	0b
+
+	strb	w15, [sp, x4]		// register first character in set
+	add	x1, x1, #2
+
+	/* process remaining chars in set */
+	.p2align 4
+
+
+0:	ldrb	w4, [x1]		// next char in set
+	strb	w15, [sp, x5]		// register previous char
+	cbz	w4, 1f			// NUL encountered?
+
+	ldrb	w5, [x1, #1]
+	add	x1, x1, #2
+	strb	w15, [sp, x4]
+	cbnz	w5, 0b
+
+1:	mov	x5, x0			// stash a copy of src
+
+	/* find mismatch */
+	.p2align 4
+0:	ldrb	w8, [x0]
+	ldrb	w9, [sp, x8]
+	cbz	w9, 2f
+
+	ldrb	w8, [x0, #1]
+	ldrb	w9, [sp, x8]
+	cbz	w9, 3f
+
+	ldrb	w8, [x0, #2]
+	ldrb	w9, [sp, x8]
+	cbz	w9, 4f
+
+	ldrb	w8, [x0, #3]
+	add	x0, x0, #4
+	ldrb	w9, [sp, x8]
+	cbnz	w9, 0b
+
+	sub	x0, x0, #3
+4:	sub	x5, x5, #1
+3:	add	x0, x0, #1
+2:	sub	x0, x0, x5
+	mov	sp, x29
+	ldp	x29, x30, [sp], #16
+	ret
+
+.Lzero:
+	mov	x0, #0
+	ret
+
+.Lsingle:
+	ldrb	w8, [x0, x5]
+	cmp	w4, w8
+	b.ne	1f
+
+	add	x5, x5, #1
+	ldrb	w8, [x0, x5]
+	cmp	w4, w8
+	b.ne	1f
+
+	add	x5, x5, #1
+	ldrb	w8, [x0, x5]
+	cmp	w4, w8
+	b.ne	1f
+
+	add	x5, x5, #1
+	ldrb	w8, [x0, x5]
+	add	x5, x5, #1
+	cmp	w4, w8
+	b.eq	.Lsingle
+
+	sub	x5, x5, #1
+1:	mov	x0, x5
+	ret
+
+END(__strspn)