diff --git a/internal/checker/checker.go b/internal/checker/checker.go
index 020fe61193..995d9f1523 100644
--- a/internal/checker/checker.go
+++ b/internal/checker/checker.go
@@ -11,7 +11,6 @@ import (
 	"strings"
 	"sync"
 	"sync/atomic"
-	"unicode/utf8"
 
 	"github.com/microsoft/typescript-go/internal/ast"
 	"github.com/microsoft/typescript-go/internal/binder"
@@ -28873,15 +28872,13 @@ func (c *Checker) getStringMappingType(symbol *ast.Symbol, t *Type) *Type {
 func applyStringMapping(symbol *ast.Symbol, str string) string {
 	switch intrinsicTypeKinds[symbol.Name] {
 	case IntrinsicTypeKindUppercase:
-		return strings.ToUpper(str)
+		return toUpperCase(str)
 	case IntrinsicTypeKindLowercase:
-		return strings.ToLower(str)
+		return toLowerCase(str)
 	case IntrinsicTypeKindCapitalize:
-		_, size := utf8.DecodeRuneInString(str)
-		return strings.ToUpper(str[:size]) + str[size:]
+		return toUpperCaseFirstRune(str)
 	case IntrinsicTypeKindUncapitalize:
-		_, size := utf8.DecodeRuneInString(str)
-		return strings.ToLower(str[:size]) + str[size:]
+		return toLowerCaseFirstRune(str)
 	}
 	return str
 }
diff --git a/internal/checker/stringcase.go b/internal/checker/stringcase.go
new file mode 100644
index 0000000000..c45d33b48b
--- /dev/null
+++ b/internal/checker/stringcase.go
@@ -0,0 +1,93 @@
+package checker
+
+import (
+	"strings"
+	"sync"
+	"unicode/utf8"
+
+	"golang.org/x/text/cases"
+	"golang.org/x/text/language"
+)
+
+// upperCaser and lowerCaser are lazily initialized via sync.OnceValue to avoid
+// paying the cost of loading golang.org/x/text/cases tables unless non-ASCII
+// input is actually encountered.
+var (
+	upperCaser = sync.OnceValue(func() *cases.Caser {
+		c := cases.Upper(language.Und)
+		return &c
+	})
+	lowerCaser = sync.OnceValue(func() *cases.Caser {
+		c := cases.Lower(language.Und)
+		return &c
+	})
+)
+
+// isASCII reports whether s contains only ASCII bytes.
+func isASCII(s string) bool {
+	for i := range len(s) {
+		if s[i] > 0x7F {
+			return false
+		}
+	}
+	return true
+}
+
+// toUpperCase converts a string to uppercase using full Unicode case mapping,
+// matching JavaScript's String.prototype.toUpperCase() behavior.
+// ASCII strings use a fast path; non-ASCII strings fall through to
+// golang.org/x/text/cases which handles special case mappings where a single
+// character maps to multiple characters (e.g., 'ß' → "SS").
+func toUpperCase(s string) string {
+	if isASCII(s) {
+		return strings.ToUpper(s)
+	}
+	return upperCaser().String(s)
+}
+
+// toLowerCase converts a string to lowercase using full Unicode case mapping,
+// matching JavaScript's String.prototype.toLowerCase() behavior.
+func toLowerCase(s string) string {
+	if isASCII(s) {
+		return strings.ToLower(s)
+	}
+	return lowerCaser().String(s)
+}
+
+// toUpperCaseFirstRune converts the first rune to uppercase using full Unicode
+// case mapping, leaving the rest of the string unchanged.
+func toUpperCaseFirstRune(s string) string {
+	if s == "" {
+		return s
+	}
+	r, size := utf8.DecodeRuneInString(s)
+	if r < 0x80 {
+		// ASCII fast path: single byte uppercase
+		if r >= 'a' && r <= 'z' {
+			b := []byte(s)
+			b[0] -= 'a' - 'A'
+			return string(b)
+		}
+		return s
+	}
+	return upperCaser().String(s[:size]) + s[size:]
+}
+
+// toLowerCaseFirstRune converts the first rune to lowercase using full Unicode
+// case mapping, leaving the rest of the string unchanged.
+func toLowerCaseFirstRune(s string) string {
+	if s == "" {
+		return s
+	}
+	r, size := utf8.DecodeRuneInString(s)
+	if r < 0x80 {
+		// ASCII fast path: single byte lowercase
+		if r >= 'A' && r <= 'Z' {
+			b := []byte(s)
+			b[0] += 'a' - 'A'
+			return string(b)
+		}
+		return s
+	}
+	return lowerCaser().String(s[:size]) + s[size:]
+}
diff --git a/testdata/baselines/reference/compiler/intrinsicTypesUnicodeSpecialCasing.js b/testdata/baselines/reference/compiler/intrinsicTypesUnicodeSpecialCasing.js
new file mode 100644
index 0000000000..e820c17a57
--- /dev/null
+++ b/testdata/baselines/reference/compiler/intrinsicTypesUnicodeSpecialCasing.js
@@ -0,0 +1,53 @@
+//// [tests/cases/compiler/intrinsicTypesUnicodeSpecialCasing.ts] ////
+
+//// [intrinsicTypesUnicodeSpecialCasing.ts]
+// Test Unicode special case mappings for intrinsic string types
+// These characters have 1:many case mappings that Go's strings.ToUpper/ToLower
+// don't handle, but JavaScript's toUpperCase()/toLowerCase() do.
+
+// ß (U+00DF) uppercases to "SS" in JavaScript
+type T1 = Uppercase<"ß">;
+const t1: T1 = "SS";
+
+// İ (U+0130) lowercases to "i̇" (i + combining dot above U+0307) in JavaScript
+type T2 = Lowercase<"İ">;
+const t2: T2 = "i\u0307";
+
+// Ligatures: fi (U+FB01) uppercases to "FI"
+type T3 = Uppercase<"fi">;
+const t3: T3 = "FI";
+
+// fl (U+FB02) uppercases to "FL"
+type T4 = Uppercase<"fl">;
+const t4: T4 = "FL";
+
+// ff (U+FB00) uppercases to "FF"
+type T5 = Uppercase<"ff">;
+const t5: T5 = "FF";
+
+// Capitalize should only affect first character
+type T6 = Capitalize<"ßtest">;
+const t6: T6 = "SStest";
+
+// Uncapitalize with İ
+type T7 = Uncapitalize<"İSPANYOL">;
+const t7: T7 = "i\u0307SPANYOL";
+
+// Mixed string with special characters
+type T8 = Uppercase<"straße">;
+const t8: T8 = "STRASSE";
+
+
+//// [intrinsicTypesUnicodeSpecialCasing.js]
+"use strict";
+// Test Unicode special case mappings for intrinsic string types
+// These characters have 1:many case mappings that Go's strings.ToUpper/ToLower
+// don't handle, but JavaScript's toUpperCase()/toLowerCase() do.
+const t1 = "SS";
+const t2 = "i\u0307";
+const t3 = "FI";
+const t4 = "FL";
+const t5 = "FF";
+const t6 = "SStest";
+const t7 = "i\u0307SPANYOL";
+const t8 = "STRASSE";
diff --git a/testdata/baselines/reference/compiler/intrinsicTypesUnicodeSpecialCasing.symbols b/testdata/baselines/reference/compiler/intrinsicTypesUnicodeSpecialCasing.symbols
new file mode 100644
index 0000000000..9ce7ca0f50
--- /dev/null
+++ b/testdata/baselines/reference/compiler/intrinsicTypesUnicodeSpecialCasing.symbols
@@ -0,0 +1,79 @@
+//// [tests/cases/compiler/intrinsicTypesUnicodeSpecialCasing.ts] ////
+
+=== intrinsicTypesUnicodeSpecialCasing.ts ===
+// Test Unicode special case mappings for intrinsic string types
+// These characters have 1:many case mappings that Go's strings.ToUpper/ToLower
+// don't handle, but JavaScript's toUpperCase()/toLowerCase() do.
+
+// ß (U+00DF) uppercases to "SS" in JavaScript
+type T1 = Uppercase<"ß">;
+>T1 : Symbol(T1, Decl(intrinsicTypesUnicodeSpecialCasing.ts, 0, 0))
+>Uppercase : Symbol(Uppercase, Decl(lib.es5.d.ts, --, --))
+
+const t1: T1 = "SS";
+>t1 : Symbol(t1, Decl(intrinsicTypesUnicodeSpecialCasing.ts, 6, 5))
+>T1 : Symbol(T1, Decl(intrinsicTypesUnicodeSpecialCasing.ts, 0, 0))
+
+// İ (U+0130) lowercases to "i̇" (i + combining dot above U+0307) in JavaScript
+type T2 = Lowercase<"İ">;
+>T2 : Symbol(T2, Decl(intrinsicTypesUnicodeSpecialCasing.ts, 6, 20))
+>Lowercase : Symbol(Lowercase, Decl(lib.es5.d.ts, --, --))
+
+const t2: T2 = "i\u0307";
+>t2 : Symbol(t2, Decl(intrinsicTypesUnicodeSpecialCasing.ts, 10, 5))
+>T2 : Symbol(T2, Decl(intrinsicTypesUnicodeSpecialCasing.ts, 6, 20))
+
+// Ligatures: fi (U+FB01) uppercases to "FI"
+type T3 = Uppercase<"fi">;
+>T3 : Symbol(T3, Decl(intrinsicTypesUnicodeSpecialCasing.ts, 10, 25))
+>Uppercase : Symbol(Uppercase, Decl(lib.es5.d.ts, --, --))
+
+const t3: T3 = "FI";
+>t3 : Symbol(t3, Decl(intrinsicTypesUnicodeSpecialCasing.ts, 14, 5))
+>T3 : Symbol(T3, Decl(intrinsicTypesUnicodeSpecialCasing.ts, 10, 25))
+
+// fl (U+FB02) uppercases to "FL"
+type T4 = Uppercase<"fl">;
+>T4 : Symbol(T4, Decl(intrinsicTypesUnicodeSpecialCasing.ts, 14, 20))
+>Uppercase : Symbol(Uppercase, Decl(lib.es5.d.ts, --, --))
+
+const t4: T4 = "FL";
+>t4 : Symbol(t4, Decl(intrinsicTypesUnicodeSpecialCasing.ts, 18, 5))
+>T4 : Symbol(T4, Decl(intrinsicTypesUnicodeSpecialCasing.ts, 14, 20))
+
+// ff (U+FB00) uppercases to "FF"
+type T5 = Uppercase<"ff">;
+>T5 : Symbol(T5, Decl(intrinsicTypesUnicodeSpecialCasing.ts, 18, 20))
+>Uppercase : Symbol(Uppercase, Decl(lib.es5.d.ts, --, --))
+
+const t5: T5 = "FF";
+>t5 : Symbol(t5, Decl(intrinsicTypesUnicodeSpecialCasing.ts, 22, 5))
+>T5 : Symbol(T5, Decl(intrinsicTypesUnicodeSpecialCasing.ts, 18, 20))
+
+// Capitalize should only affect first character
+type T6 = Capitalize<"ßtest">;
+>T6 : Symbol(T6, Decl(intrinsicTypesUnicodeSpecialCasing.ts, 22, 20))
+>Capitalize : Symbol(Capitalize, Decl(lib.es5.d.ts, --, --))
+
+const t6: T6 = "SStest";
+>t6 : Symbol(t6, Decl(intrinsicTypesUnicodeSpecialCasing.ts, 26, 5))
+>T6 : Symbol(T6, Decl(intrinsicTypesUnicodeSpecialCasing.ts, 22, 20))
+
+// Uncapitalize with İ
+type T7 = Uncapitalize<"İSPANYOL">;
+>T7 : Symbol(T7, Decl(intrinsicTypesUnicodeSpecialCasing.ts, 26, 24))
+>Uncapitalize : Symbol(Uncapitalize, Decl(lib.es5.d.ts, --, --))
+
+const t7: T7 = "i\u0307SPANYOL";
+>t7 : Symbol(t7, Decl(intrinsicTypesUnicodeSpecialCasing.ts, 30, 5))
+>T7 : Symbol(T7, Decl(intrinsicTypesUnicodeSpecialCasing.ts, 26, 24))
+
+// Mixed string with special characters
+type T8 = Uppercase<"straße">;
+>T8 : Symbol(T8, Decl(intrinsicTypesUnicodeSpecialCasing.ts, 30, 32))
+>Uppercase : Symbol(Uppercase, Decl(lib.es5.d.ts, --, --))
+
+const t8: T8 = "STRASSE";
+>t8 : Symbol(t8, Decl(intrinsicTypesUnicodeSpecialCasing.ts, 34, 5))
+>T8 : Symbol(T8, Decl(intrinsicTypesUnicodeSpecialCasing.ts, 30, 32))
+
diff --git a/testdata/baselines/reference/compiler/intrinsicTypesUnicodeSpecialCasing.types b/testdata/baselines/reference/compiler/intrinsicTypesUnicodeSpecialCasing.types
new file mode 100644
index 0000000000..e8b7342e2b
--- /dev/null
+++ b/testdata/baselines/reference/compiler/intrinsicTypesUnicodeSpecialCasing.types
@@ -0,0 +1,71 @@
+//// [tests/cases/compiler/intrinsicTypesUnicodeSpecialCasing.ts] ////
+
+=== intrinsicTypesUnicodeSpecialCasing.ts ===
+// Test Unicode special case mappings for intrinsic string types
+// These characters have 1:many case mappings that Go's strings.ToUpper/ToLower
+// don't handle, but JavaScript's toUpperCase()/toLowerCase() do.
+
+// ß (U+00DF) uppercases to "SS" in JavaScript
+type T1 = Uppercase<"ß">;
+>T1 : "SS"
+
+const t1: T1 = "SS";
+>t1 : "SS"
+>"SS" : "SS"
+
+// İ (U+0130) lowercases to "i̇" (i + combining dot above U+0307) in JavaScript
+type T2 = Lowercase<"İ">;
+>T2 : "i̇"
+
+const t2: T2 = "i\u0307";
+>t2 : "i̇"
+>"i\u0307" : "i̇"
+
+// Ligatures: fi (U+FB01) uppercases to "FI"
+type T3 = Uppercase<"fi">;
+>T3 : "FI"
+
+const t3: T3 = "FI";
+>t3 : "FI"
+>"FI" : "FI"
+
+// fl (U+FB02) uppercases to "FL"
+type T4 = Uppercase<"fl">;
+>T4 : "FL"
+
+const t4: T4 = "FL";
+>t4 : "FL"
+>"FL" : "FL"
+
+// ff (U+FB00) uppercases to "FF"
+type T5 = Uppercase<"ff">;
+>T5 : "FF"
+
+const t5: T5 = "FF";
+>t5 : "FF"
+>"FF" : "FF"
+
+// Capitalize should only affect first character
+type T6 = Capitalize<"ßtest">;
+>T6 : "SStest"
+
+const t6: T6 = "SStest";
+>t6 : "SStest"
+>"SStest" : "SStest"
+
+// Uncapitalize with İ
+type T7 = Uncapitalize<"İSPANYOL">;
+>T7 : "i̇SPANYOL"
+
+const t7: T7 = "i\u0307SPANYOL";
+>t7 : "i̇SPANYOL"
+>"i\u0307SPANYOL" : "i̇SPANYOL"
+
+// Mixed string with special characters
+type T8 = Uppercase<"straße">;
+>T8 : "STRASSE"
+
+const t8: T8 = "STRASSE";
+>t8 : "STRASSE"
+>"STRASSE" : "STRASSE"
+
diff --git a/testdata/tests/cases/compiler/intrinsicTypesUnicodeSpecialCasing.ts b/testdata/tests/cases/compiler/intrinsicTypesUnicodeSpecialCasing.ts
new file mode 100644
index 0000000000..d1a50f563f
--- /dev/null
+++ b/testdata/tests/cases/compiler/intrinsicTypesUnicodeSpecialCasing.ts
@@ -0,0 +1,37 @@
+// @strict: true
+
+// Test Unicode special case mappings for intrinsic string types
+// These characters have 1:many case mappings that Go's strings.ToUpper/ToLower
+// don't handle, but JavaScript's toUpperCase()/toLowerCase() do.
+
+// ß (U+00DF) uppercases to "SS" in JavaScript
+type T1 = Uppercase<"ß">;
+const t1: T1 = "SS";
+
+// İ (U+0130) lowercases to "i̇" (i + combining dot above U+0307) in JavaScript
+type T2 = Lowercase<"İ">;
+const t2: T2 = "i\u0307";
+
+// Ligatures: fi (U+FB01) uppercases to "FI"
+type T3 = Uppercase<"fi">;
+const t3: T3 = "FI";
+
+// fl (U+FB02) uppercases to "FL"
+type T4 = Uppercase<"fl">;
+const t4: T4 = "FL";
+
+// ff (U+FB00) uppercases to "FF"
+type T5 = Uppercase<"ff">;
+const t5: T5 = "FF";
+
+// Capitalize should only affect first character
+type T6 = Capitalize<"ßtest">;
+const t6: T6 = "SStest";
+
+// Uncapitalize with İ
+type T7 = Uncapitalize<"İSPANYOL">;
+const t7: T7 = "i\u0307SPANYOL";
+
+// Mixed string with special characters
+type T8 = Uppercase<"straße">;
+const t8: T8 = "STRASSE";