diff --git a/src/unicode/utf8/utf8.go b/src/unicode/utf8/utf8.go
index c7389d4d6f5..1c1391d55b5 100644
--- a/src/unicode/utf8/utf8.go
+++ b/src/unicode/utf8/utf8.go
@@ -61,6 +61,12 @@ const (
 	s7 = 0x44 // accept 4, size 4
 )
 
+const (
+	runeErrorByte0 = t3 | (RuneError >> 12)
+	runeErrorByte1 = tx | (RuneError>>6)&maskx
+	runeErrorByte2 = tx | RuneError&maskx
+)
+
 // first is information about the first byte in a UTF-8 sequence.
 var first = [256]uint8{
 	//   1   2   3   4   5   6   7   8   9   A   B   C   D   E   F
@@ -340,32 +346,41 @@ func RuneLen(r rune) int {
 // If the rune is out of range, it writes the encoding of [RuneError].
 // It returns the number of bytes written.
 func EncodeRune(p []byte, r rune) int {
-	// Negative values are erroneous. Making it unsigned addresses the problem.
-	switch i := uint32(r); {
-	case i <= rune1Max:
+	// This function is inlineable for fast handling of ASCII.
+	if uint32(r) <= rune1Max {
 		p[0] = byte(r)
 		return 1
+	}
+	return encodeRuneNonASCII(p, r)
+}
+
+func encodeRuneNonASCII(p []byte, r rune) int {
+	// Negative values are erroneous. Making it unsigned addresses the problem.
+	switch i := uint32(r); {
 	case i <= rune2Max:
 		_ = p[1] // eliminate bounds checks
 		p[0] = t2 | byte(r>>6)
 		p[1] = tx | byte(r)&maskx
 		return 2
-	case i > MaxRune, surrogateMin <= i && i <= surrogateMax:
-		r = RuneError
-		fallthrough
-	case i <= rune3Max:
+	case i < surrogateMin, surrogateMax < i && i <= rune3Max:
 		_ = p[2] // eliminate bounds checks
 		p[0] = t3 | byte(r>>12)
 		p[1] = tx | byte(r>>6)&maskx
 		p[2] = tx | byte(r)&maskx
 		return 3
-	default:
+	case i > rune3Max && i <= MaxRune:
 		_ = p[3] // eliminate bounds checks
 		p[0] = t4 | byte(r>>18)
 		p[1] = tx | byte(r>>12)&maskx
 		p[2] = tx | byte(r>>6)&maskx
 		p[3] = tx | byte(r)&maskx
 		return 4
+	default:
+		_ = p[2] // eliminate bounds checks
+		p[0] = runeErrorByte0
+		p[1] = runeErrorByte1
+		p[2] = runeErrorByte2
+		return 3
 	}
 }
 
@@ -385,13 +400,12 @@ func appendRuneNonASCII(p []byte, r rune) []byte {
 	switch i := uint32(r); {
 	case i <= rune2Max:
 		return append(p, t2|byte(r>>6), tx|byte(r)&maskx)
-	case i > MaxRune, surrogateMin <= i && i <= surrogateMax:
-		r = RuneError
-		fallthrough
-	case i <= rune3Max:
+	case i < surrogateMin, surrogateMax < i && i <= rune3Max:
 		return append(p, t3|byte(r>>12), tx|byte(r>>6)&maskx, tx|byte(r)&maskx)
-	default:
+	case i > rune3Max && i <= MaxRune:
 		return append(p, t4|byte(r>>18), tx|byte(r>>12)&maskx, tx|byte(r>>6)&maskx, tx|byte(r)&maskx)
+	default:
+		return append(p, runeErrorByte0, runeErrorByte1, runeErrorByte2)
 	}
 }
 
diff --git a/src/unicode/utf8/utf8_test.go b/src/unicode/utf8/utf8_test.go
index 19a04dc92e7..fa23419b364 100644
--- a/src/unicode/utf8/utf8_test.go
+++ b/src/unicode/utf8/utf8_test.go
@@ -641,28 +641,98 @@ func init() {
 func BenchmarkEncodeASCIIRune(b *testing.B) {
 	buf := make([]byte, UTFMax)
 	for i := 0; i < b.N; i++ {
-		EncodeRune(buf, 'a')
+		EncodeRune(buf, 'a') // 1 byte
+	}
+}
+
+func BenchmarkEncodeSpanishRune(b *testing.B) {
+	buf := make([]byte, UTFMax)
+	for i := 0; i < b.N; i++ {
+		EncodeRune(buf, 'Ñ') // 2 bytes
 	}
 }
 
 func BenchmarkEncodeJapaneseRune(b *testing.B) {
 	buf := make([]byte, UTFMax)
 	for i := 0; i < b.N; i++ {
-		EncodeRune(buf, '本')
+		EncodeRune(buf, '本') // 3 bytes
+	}
+}
+
+func BenchmarkEncodeMaxRune(b *testing.B) {
+	buf := make([]byte, UTFMax)
+	for i := 0; i < b.N; i++ {
+		EncodeRune(buf, MaxRune) // 4 bytes
+	}
+}
+
+func BenchmarkEncodeInvalidRuneMaxPlusOne(b *testing.B) {
+	buf := make([]byte, UTFMax)
+	for i := 0; i < b.N; i++ {
+		EncodeRune(buf, MaxRune+1) // 3 bytes: RuneError
+	}
+}
+
+func BenchmarkEncodeInvalidRuneSurrogate(b *testing.B) {
+	buf := make([]byte, UTFMax)
+	for i := 0; i < b.N; i++ {
+		EncodeRune(buf, 0xD800) // 3 bytes: RuneError
+	}
+}
+
+func BenchmarkEncodeInvalidRuneNegative(b *testing.B) {
+	buf := make([]byte, UTFMax)
+	for i := 0; i < b.N; i++ {
+		EncodeRune(buf, -1) // 3 bytes: RuneError
 	}
 }
 
 func BenchmarkAppendASCIIRune(b *testing.B) {
 	buf := make([]byte, UTFMax)
 	for i := 0; i < b.N; i++ {
-		AppendRune(buf[:0], 'a')
+		AppendRune(buf[:0], 'a') // 1 byte
+	}
+}
+
+func BenchmarkAppendSpanishRune(b *testing.B) {
+	buf := make([]byte, UTFMax)
+	for i := 0; i < b.N; i++ {
+		AppendRune(buf[:0], 'Ñ') // 2 bytes
 	}
 }
 
 func BenchmarkAppendJapaneseRune(b *testing.B) {
 	buf := make([]byte, UTFMax)
 	for i := 0; i < b.N; i++ {
-		AppendRune(buf[:0], '本')
+		AppendRune(buf[:0], '本') // 3 bytes
+	}
+}
+
+func BenchmarkAppendMaxRune(b *testing.B) {
+	buf := make([]byte, UTFMax)
+	for i := 0; i < b.N; i++ {
+		AppendRune(buf[:0], MaxRune) // 4 bytes
+	}
+}
+
+func BenchmarkAppendInvalidRuneMaxPlusOne(b *testing.B) {
+	buf := make([]byte, UTFMax)
+	for i := 0; i < b.N; i++ {
+		AppendRune(buf[:0], MaxRune+1) // 3 bytes: RuneError
+	}
+}
+
+func BenchmarkAppendInvalidRuneSurrogate(b *testing.B) {
+	buf := make([]byte, UTFMax)
+	for i := 0; i < b.N; i++ {
+		AppendRune(buf[:0], 0xD800) // 3 bytes: RuneError
+	}
+}
+
+func BenchmarkAppendInvalidRuneNegative(b *testing.B) {
+	buf := make([]byte, UTFMax)
+	for i := 0; i < b.N; i++ {
+		AppendRune(buf[:0], -1) // 3 bytes: RuneError
 	}
 }