diff --git a/internal/poly1305/mac_noasm.go b/internal/poly1305/mac_noasm.go
index 8d99551..b1da456 100644
--- a/internal/poly1305/mac_noasm.go
+++ b/internal/poly1305/mac_noasm.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build (!amd64 && !loong64 && !ppc64le && !ppc64 && !s390x) || !gc || purego
+//go:build (!amd64 && !loong64 && !ppc64le && !ppc64 && !riscv64 && !s390x) || !gc || purego
 
 package poly1305
 
diff --git a/internal/poly1305/sum_asm.go b/internal/poly1305/sum_asm.go
index 315b84a..55041bf 100644
--- a/internal/poly1305/sum_asm.go
+++ b/internal/poly1305/sum_asm.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build gc && !purego && (amd64 || loong64 || ppc64 || ppc64le)
+//go:build gc && !purego && (amd64 || loong64 || ppc64 || ppc64le || riscv64)
 
 package poly1305
 
diff --git a/internal/poly1305/sum_riscv64.s b/internal/poly1305/sum_riscv64.s
new file mode 100644
index 0000000..ce5eb3d
--- /dev/null
+++ b/internal/poly1305/sum_riscv64.s
@@ -0,0 +1,158 @@
+// Copyright 2026 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build gc && !purego
+
+#define LOAD64U(base, offset, t0, t1, t2, t3, dst) \
+	MOVBU	(offset+0*1)(base), t0; \
+	MOVBU	(offset+1*1)(base), t1; \
+	MOVBU	(offset+2*1)(base), t2; \
+	MOVBU	(offset+3*1)(base), t3; \
+	SLL	$8, t1; \
+	SLL	$16, t2; \
+	SLL	$24, t3; \
+	OR	t1, t0; \
+	OR	t3, t2; \
+	OR	t2, t0, dst; \
+	MOVBU	(offset+4*1)(base), t0; \
+	MOVBU	(offset+5*1)(base), t1; \
+	MOVBU	(offset+6*1)(base), t2; \
+	MOVBU	(offset+7*1)(base), t3; \
+	SLL	$32, t0; \
+	SLL	$40, t1; \
+	SLL	$48, t2; \
+	SLL	$56, t3; \
+	OR	t1, t0; \
+	OR	t3, t2; \
+	OR	t2, t0; \
+	OR	t0, dst
+
+// func update(state *macState, msg []byte)
+TEXT ·update(SB), $0-32
+	MOV	state+0(FP), X5
+	MOV	msg_base+8(FP), X6
+	MOV	msg_len+16(FP), X7
+
+	MOV	$16, X8
+
+	AND	$7, X6, X28
+
+	MOV	(0*8)(X5), X9		// h0
+	MOV	(1*8)(X5), X10		// h1
+	MOV	(2*8)(X5), X11		// h2
+	MOV	(3*8)(X5), X12		// r0
+	MOV	(4*8)(X5), X13		// r1
+
+	BLT	X7, X8, tail
+
+loop:
+	BEQZ	X28, aligned_load
+
+	LOAD64U(X6, 0*8, X16, X18, X19, X20, X15)	// msg[0:8]
+	LOAD64U(X6, 1*8, X16, X18, X19, X20, X17)	// msg[8:16]
+	JMP	block
+
+aligned_load:
+	MOV	(0*8)(X6), X15		// msg[0:8]
+	MOV	(1*8)(X6), X17		// msg[8:16]
+
+block:
+	ADD	X15, X9		// h0 (x1 + y1 = z1', if z1' < x1 then z1' overflow)
+	SLTU	X15, X9, X19	// h0.carry
+	ADD	X17, X10, X22
+	SLTU	X17, X22, X23
+	ADD	X22, X19, X10	// h1
+	SLTU	X22, X10, X19
+	OR	X23, X19	// h1.carry
+	ADD	$1, X19
+	ADD	X19, X11	// h2
+
+	ADD	$16, X6		// msg = msg[16:]
+
+multiply:
+	MULHU	X9, X12, X16	// h0r0.hi
+	MUL	X9, X12, X15	// h0r0.lo
+	MULHU	X10, X12, X17	// h1r0.hi
+	MUL	X10, X12, X14	// h1r0.lo
+	ADD	X14, X16
+	SLTU	X14, X16, X19
+	ADD	X19, X17
+	MUL	X11, X12, X20
+	ADD	X17, X20
+	MULHU	X9, X13, X17	// h0r1.hi
+	MUL	X9, X13, X14	// h0r1.lo
+	ADD	X14, X16
+	SLTU	X14, X16, X19
+	ADD	X19, X17
+	MOV	X17, X9
+	MUL	X11, X13, X21	// h2r1
+	MULHU	X10, X13, X17	// h1r1.hi
+	MUL	X10, X13, X14	// h1r1.lo
+	ADD	X14, X20
+	ADD	X17, X21, X22
+	SLTU	X14, X20, X19
+	ADD	X22, X19, X21
+	ADD	X9, X20
+	SLTU	X9, X20, X19
+	ADD	X19, X21
+	AND	$3, X20, X11
+	AND	$-4, X20, X18
+	ADD	X18, X15, X9
+	ADD	X21, X16, X22
+	SLTU	X18, X9, X19
+	SLTU	X21, X22, X23
+	ADD	X22, X19, X10
+	SLTU	X22, X10, X19
+	OR	X19, X23, X19
+	ADD	X19, X11
+	SLL	$62, X21, X22
+	SRL	$2, X20, X23
+	SRL	$2, X21, X21
+	OR	X22, X23, X20
+	ADD	X20, X9, X9
+	ADD	X21, X10, X22
+	SLTU	X20, X9, X19
+	SLTU	X21, X22, X23
+	ADD	X22, X19, X10
+	SLTU	X22, X10, X19
+	OR	X19, X23, X19
+	ADD	X19, X11, X11
+
+	SUB	$16, X7, X7
+	BGE	X7, X8, loop
+
+tail:
+	BEQ	X7, X0, done
+	MOV	$1, X15
+	MOV $0, X16
+	ADD	X7, X6, X6
+
+flush_buffer:
+	MOVBU	-1(X6), X20
+	SRL	$56, X15, X19
+	SLL	$8, X16, X23
+	SLL	$8, X15, X15
+	OR	X19, X23, X16
+	XOR	X20, X15
+	SUB	$1, X7, X7
+	SUB	$1, X6, X6
+	BNE	X7, X0, flush_buffer
+
+	ADD	X15, X9
+	SLTU	X15, X9, X19
+	ADD	X16, X10, X22
+	SLTU	X16, X22, X23
+	ADD	X22, X19, X10
+	SLTU	X22, X10, X19
+	OR	X23, X19
+	ADD	X19, X11
+
+	MOV	$16, X7
+	JMP	multiply
+
+done:
+	MOV	X9, (0*8)(X5)	// h0
+	MOV	X10, (1*8)(X5)
+	MOV	X11, (2*8)(X5)
+	RET