diff --git a/src/encoding/json/decode.go b/src/encoding/json/decode.go
index 0a1cadf42e..53470d8c88 100644
--- a/src/encoding/json/decode.go
+++ b/src/encoding/json/decode.go
@@ -690,20 +690,9 @@ func (d *decodeState) object(v reflect.Value) error {
 			}
 			subv = mapElem
 		} else {
-			var f *field
-			if i, ok := fields.nameIndex[string(key)]; ok {
-				// Found an exact name match.
-				f = &fields.list[i]
-			} else {
-				// Fall back to the expensive case-insensitive
-				// linear search.
-				for i := range fields.list {
-					ff := &fields.list[i]
-					if ff.equalFold(ff.nameBytes, key) {
-						f = ff
-						break
-					}
-				}
+			f := fields.byExactName[string(key)]
+			if f == nil {
+				f = fields.byFoldedName[string(foldName(key))]
 			}
 			if f != nil {
 				subv = v
diff --git a/src/encoding/json/encode.go b/src/encoding/json/encode.go
index de639aa008..f3c824d13e 100644
--- a/src/encoding/json/encode.go
+++ b/src/encoding/json/encode.go
@@ -672,8 +672,9 @@ type structEncoder struct {
 }
 
 type structFields struct {
-	list      []field
-	nameIndex map[string]int
+	list         []field
+	byExactName  map[string]*field
+	byFoldedName map[string]*field
 }
 
 func (se structEncoder) encode(e *encodeState, v reflect.Value, opts encOpts) {
@@ -1033,8 +1034,7 @@ func appendString[Bytes []byte | string](dst []byte, src Bytes, escapeHTML bool)
 // A field represents a single field found in a struct.
 type field struct {
 	name      string
-	nameBytes []byte                 // []byte(name)
-	equalFold func(s, t []byte) bool // bytes.EqualFold or equivalent
+	nameBytes []byte // []byte(name)
 
 	nameNonEsc  string // `"` + name + `":`
 	nameEscHTML string // `"` + HTMLEscape(name) + `":`
@@ -1161,7 +1161,6 @@ func typeFields(t reflect.Type) structFields {
 						quoted:    quoted,
 					}
 					field.nameBytes = []byte(field.name)
-					field.equalFold = foldFunc(field.nameBytes)
 
 					// Build nameEscHTML and nameNonEsc ahead of time.
 					nameEscBuf = appendHTMLEscape(nameEscBuf[:0], field.nameBytes)
@@ -1240,11 +1239,16 @@ func typeFields(t reflect.Type) structFields {
 		f := &fields[i]
 		f.encoder = typeEncoder(typeByIndex(t, f.index))
 	}
-	nameIndex := make(map[string]int, len(fields))
+	exactNameIndex := make(map[string]*field, len(fields))
+	foldedNameIndex := make(map[string]*field, len(fields))
 	for i, field := range fields {
-		nameIndex[field.name] = i
+		exactNameIndex[field.name] = &fields[i]
+		// For historical reasons, first folded match takes precedence.
+		if _, ok := foldedNameIndex[string(foldName(field.nameBytes))]; !ok {
+			foldedNameIndex[string(foldName(field.nameBytes))] = &fields[i]
+		}
 	}
-	return structFields{fields, nameIndex}
+	return structFields{fields, exactNameIndex, foldedNameIndex}
 }
 
 // dominantField looks through the fields, all of which are known to
diff --git a/src/encoding/json/fold.go b/src/encoding/json/fold.go
index 0f9b09d712..c4c671b527 100644
--- a/src/encoding/json/fold.go
+++ b/src/encoding/json/fold.go
@@ -5,137 +5,44 @@
 package json
 
 import (
-	"bytes"
+	"unicode"
 	"unicode/utf8"
 )
 
-const (
-	caseMask     = ^byte(0x20) // Mask to ignore case in ASCII.
-	kelvin       = '\u212a'
-	smallLongEss = '\u017f'
-)
-
-// foldFunc returns one of four different case folding equivalence
-// functions, from most general (and slow) to fastest:
-//
-// 1) bytes.EqualFold, if the key s contains any non-ASCII UTF-8
-// 2) equalFoldRight, if s contains special folding ASCII ('k', 'K', 's', 'S')
-// 3) asciiEqualFold, no special, but includes non-letters (including _)
-// 4) simpleLetterEqualFold, no specials, no non-letters.
-//
-// The letters S and K are special because they map to 3 runes, not just 2:
-//   - S maps to s and to U+017F 'ſ' Latin small letter long s
-//   - k maps to K and to U+212A 'K' Kelvin sign
-//
-// See https://play.golang.org/p/tTxjOc0OGo
-//
-// The returned function is specialized for matching against s and
-// should only be given s. It's not curried for performance reasons.
-func foldFunc(s []byte) func(s, t []byte) bool {
-	nonLetter := false
-	special := false // special letter
-	for _, b := range s {
-		if b >= utf8.RuneSelf {
-			return bytes.EqualFold
-		}
-		upper := b & caseMask
-		if upper < 'A' || upper > 'Z' {
-			nonLetter = true
-		} else if upper == 'K' || upper == 'S' {
-			// See above for why these letters are special.
-			special = true
-		}
-	}
-	if special {
-		return equalFoldRight
-	}
-	if nonLetter {
-		return asciiEqualFold
-	}
-	return simpleLetterEqualFold
+// foldName returns a folded string such that foldName(x) == foldName(y)
+// is identical to bytes.EqualFold(x, y).
+func foldName(in []byte) []byte {
+	// This is inlinable to take advantage of "function outlining".
+	var arr [32]byte // large enough for most JSON names
+	return appendFoldedName(arr[:0], in)
 }
 
-// equalFoldRight is a specialization of bytes.EqualFold when s is
-// known to be all ASCII (including punctuation), but contains an 's',
-// 'S', 'k', or 'K', requiring a Unicode fold on the bytes in t.
-// See comments on foldFunc.
-func equalFoldRight(s, t []byte) bool {
-	for _, sb := range s {
-		if len(t) == 0 {
-			return false
-		}
-		tb := t[0]
-		if tb < utf8.RuneSelf {
-			if sb != tb {
-				sbUpper := sb & caseMask
-				if 'A' <= sbUpper && sbUpper <= 'Z' {
-					if sbUpper != tb&caseMask {
-						return false
-					}
-				} else {
-					return false
-				}
+func appendFoldedName(out, in []byte) []byte {
+	for i := 0; i < len(in); {
+		// Handle single-byte ASCII.
+		if c := in[i]; c < utf8.RuneSelf {
+			if 'a' <= c && c <= 'z' {
+				c -= 'a' - 'A'
 			}
-			t = t[1:]
+			out = append(out, c)
+			i++
 			continue
 		}
-		// sb is ASCII and t is not. t must be either kelvin
-		// sign or long s; sb must be s, S, k, or K.
-		tr, size := utf8.DecodeRune(t)
-		switch sb {
-		case 's', 'S':
-			if tr != smallLongEss {
-				return false
-			}
-		case 'k', 'K':
-			if tr != kelvin {
-				return false
-			}
-		default:
-			return false
-		}
-		t = t[size:]
-
+		// Handle multi-byte Unicode.
+		r, n := utf8.DecodeRune(in[i:])
+		out = utf8.AppendRune(out, foldRune(r))
+		i += n
 	}
-	return len(t) == 0
+	return out
 }
 
-// asciiEqualFold is a specialization of bytes.EqualFold for use when
-// s is all ASCII (but may contain non-letters) and contains no
-// special-folding letters.
-// See comments on foldFunc.
-func asciiEqualFold(s, t []byte) bool {
-	if len(s) != len(t) {
-		return false
-	}
-	for i, sb := range s {
-		tb := t[i]
-		if sb == tb {
-			continue
-		}
-		if ('a' <= sb && sb <= 'z') || ('A' <= sb && sb <= 'Z') {
-			if sb&caseMask != tb&caseMask {
-				return false
-			}
-		} else {
-			return false
+// foldRune is returns the smallest rune for all runes in the same fold set.
+func foldRune(r rune) rune {
+	for {
+		r2 := unicode.SimpleFold(r)
+		if r2 <= r {
+			return r2
 		}
+		r = r2
 	}
-	return true
-}
-
-// simpleLetterEqualFold is a specialization of bytes.EqualFold for
-// use when s is all ASCII letters (no underscores, etc) and also
-// doesn't contain 'k', 'K', 's', or 'S'.
-// See comments on foldFunc.
-func simpleLetterEqualFold(s, t []byte) bool {
-	if len(s) != len(t) {
-		return false
-	}
-	for i, b := range s {
-		if b&caseMask != t[i]&caseMask {
-			return false
-		}
-	}
-	return true
 }
diff --git a/src/encoding/json/fold_test.go b/src/encoding/json/fold_test.go
index 4daa3590f5..9d6fd0559d 100644
--- a/src/encoding/json/fold_test.go
+++ b/src/encoding/json/fold_test.go
@@ -6,105 +6,45 @@ package json
 
 import (
 	"bytes"
-	"strings"
 	"testing"
-	"unicode/utf8"
 )
 
-var foldTests = []struct {
-	fn   func(s, t []byte) bool
-	s, t string
-	want bool
-}{
-	{equalFoldRight, "", "", true},
-	{equalFoldRight, "a", "a", true},
-	{equalFoldRight, "", "a", false},
-	{equalFoldRight, "a", "", false},
-	{equalFoldRight, "a", "A", true},
-	{equalFoldRight, "AB", "ab", true},
-	{equalFoldRight, "AB", "ac", false},
-	{equalFoldRight, "sbkKc", "ſbKKc", true},
-	{equalFoldRight, "SbKkc", "ſbKKc", true},
-	{equalFoldRight, "SbKkc", "ſbKK", false},
-	{equalFoldRight, "e", "é", false},
-	{equalFoldRight, "s", "S", true},
-
-	{simpleLetterEqualFold, "", "", true},
-	{simpleLetterEqualFold, "abc", "abc", true},
-	{simpleLetterEqualFold, "abc", "ABC", true},
-	{simpleLetterEqualFold, "abc", "ABCD", false},
-	{simpleLetterEqualFold, "abc", "xxx", false},
-
-	{asciiEqualFold, "a_B", "A_b", true},
-	{asciiEqualFold, "aa@", "aa`", false}, // verify 0x40 and 0x60 aren't case-equivalent
-}
-
-func TestFold(t *testing.T) {
-	for i, tt := range foldTests {
-		if got := tt.fn([]byte(tt.s), []byte(tt.t)); got != tt.want {
-			t.Errorf("%d. %q, %q = %v; want %v", i, tt.s, tt.t, got, tt.want)
+func FuzzEqualFold(f *testing.F) {
+	for _, ss := range [][2]string{
+		{"", ""},
+		{"123abc", "123ABC"},
+		{"αβδ", "ΑΒΔ"},
+		{"abc", "xyz"},
+		{"abc", "XYZ"},
+		{"1", "2"},
+		{"hello, world!", "hello, world!"},
+		{"hello, world!", "Hello, World!"},
+		{"hello, world!", "HELLO, WORLD!"},
+		{"hello, world!", "jello, world!"},
+		{"γειά, κόσμε!", "γειά, κόσμε!"},
+		{"γειά, κόσμε!", "Γειά, Κόσμε!"},
+		{"γειά, κόσμε!", "ΓΕΙΆ, ΚΌΣΜΕ!"},
+		{"γειά, κόσμε!", "ΛΕΙΆ, ΚΌΣΜΕ!"},
+		{"AESKey", "aesKey"},
+		{"AESKEY", "aes_key"},
+		{"aes_key", "AES_KEY"},
+		{"AES_KEY", "aes-key"},
+		{"aes-key", "AES-KEY"},
+		{"AES-KEY", "aesKey"},
+		{"aesKey", "AesKey"},
+		{"AesKey", "AESKey"},
+		{"AESKey", "aeskey"},
+		{"DESKey", "aeskey"},
+		{"AES Key", "aeskey"},
+	} {
+		f.Add([]byte(ss[0]), []byte(ss[1]))
+	}
+	equalFold := func(x, y []byte) bool { return string(foldName(x)) == string(foldName(y)) }
+	f.Fuzz(func(t *testing.T, x, y []byte) {
+		got := equalFold(x, y)
+		want := bytes.EqualFold(x, y)
+		if got != want {
+			t.Errorf("equalFold(%q, %q) = %v, want %v", x, y, got, want)
 		}
-		truth := strings.EqualFold(tt.s, tt.t)
-		if truth != tt.want {
-			t.Errorf("strings.EqualFold doesn't agree with case %d", i)
-		}
-	}
-}
-
-func TestFoldAgainstUnicode(t *testing.T) {
-	var buf1, buf2 []byte
-	var runes []rune
-	for i := 0x20; i <= 0x7f; i++ {
-		runes = append(runes, rune(i))
-	}
-	runes = append(runes, kelvin, smallLongEss)
-
-	funcs := []struct {
-		name   string
-		fold   func(s, t []byte) bool
-		letter bool // must be ASCII letter
-		simple bool // must be simple ASCII letter (not 'S' or 'K')
-	}{
-		{
-			name: "equalFoldRight",
-			fold: equalFoldRight,
-		},
-		{
-			name:   "asciiEqualFold",
-			fold:   asciiEqualFold,
-			simple: true,
-		},
-		{
-			name:   "simpleLetterEqualFold",
-			fold:   simpleLetterEqualFold,
-			simple: true,
-			letter: true,
-		},
-	}
-
-	for _, ff := range funcs {
-		for _, r := range runes {
-			if r >= utf8.RuneSelf {
-				continue
-			}
-			if ff.letter && !isASCIILetter(byte(r)) {
-				continue
-			}
-			if ff.simple && (r == 's' || r == 'S' || r == 'k' || r == 'K') {
-				continue
-			}
-			for _, r2 := range runes {
-				buf1 = append(utf8.AppendRune(append(buf1[:0], 'x'), r), 'x')
-				buf2 = append(utf8.AppendRune(append(buf2[:0], 'x'), r2), 'x')
-				want := bytes.EqualFold(buf1, buf2)
-				if got := ff.fold(buf1, buf2); got != want {
-					t.Errorf("%s(%q, %q) = %v; want %v", ff.name, buf1, buf2, got, want)
-				}
-			}
-		}
-	}
-}
-
-func isASCIILetter(b byte) bool {
-	return ('A' <= b && b <= 'Z') || ('a' <= b && b <= 'z')
+	})
 }