diff --git a/Userland/Libraries/LibJS/CMakeLists.txt b/Userland/Libraries/LibJS/CMakeLists.txt
index 344f0d2864..17967c78a5 100644
--- a/Userland/Libraries/LibJS/CMakeLists.txt
+++ b/Userland/Libraries/LibJS/CMakeLists.txt
@@ -165,4 +165,4 @@ set(SOURCES
 )
 
 serenity_lib(LibJS js)
-target_link_libraries(LibJS LibM LibCore LibCrypto LibRegex LibSyntax)
+target_link_libraries(LibJS LibM LibCore LibCrypto LibRegex LibSyntax LibUnicode)
diff --git a/Userland/Libraries/LibJS/Runtime/StringPrototype.cpp b/Userland/Libraries/LibJS/Runtime/StringPrototype.cpp
index 7ce3d70df7..08e3ea63d8 100644
--- a/Userland/Libraries/LibJS/Runtime/StringPrototype.cpp
+++ b/Userland/Libraries/LibJS/Runtime/StringPrototype.cpp
@@ -20,6 +20,7 @@
 #include <LibJS/Runtime/StringObject.h>
 #include <LibJS/Runtime/StringPrototype.h>
 #include <LibJS/Runtime/Value.h>
+#include <LibUnicode/CharacterTypes.h>
 #include <string.h>
 
 namespace JS {
@@ -380,21 +381,33 @@ JS_DEFINE_NATIVE_FUNCTION(StringPrototype::index_of)
 // 22.1.3.26 String.prototype.toLowerCase ( ), https://tc39.es/ecma262/#sec-string.prototype.tolowercase
 JS_DEFINE_NATIVE_FUNCTION(StringPrototype::to_lowercase)
 {
-    // FIXME: Implement Unicode case folding: https://www.unicode.org/Public/13.0.0/ucd/CaseFolding.txt
-    auto string = ak_string_from(vm, global_object);
-    if (!string.has_value())
+    auto string = utf16_string_from(vm, global_object);
+    if (vm.exception())
         return {};
-    return js_string(vm, string->to_lowercase());
+
+    Utf16View utf16_string_view { string };
+    StringBuilder builder;
+
+    for (auto code_point : utf16_string_view)
+        builder.append_code_point(Unicode::to_unicode_lowercase(code_point));
+
+    return js_string(vm, builder.to_string());
 }
 
 // 22.1.3.28 String.prototype.toUpperCase ( ), https://tc39.es/ecma262/#sec-string.prototype.touppercase
 JS_DEFINE_NATIVE_FUNCTION(StringPrototype::to_uppercase)
 {
-    // FIXME: Implement Unicode case folding: https://www.unicode.org/Public/13.0.0/ucd/CaseFolding.txt
-    auto string = ak_string_from(vm, global_object);
-    if (!string.has_value())
+    auto string = utf16_string_from(vm, global_object);
+    if (vm.exception())
         return {};
-    return js_string(vm, string->to_uppercase());
+
+    Utf16View utf16_string_view { string };
+    StringBuilder builder;
+
+    for (auto code_point : utf16_string_view)
+        builder.append_code_point(Unicode::to_unicode_uppercase(code_point));
+
+    return js_string(vm, builder.to_string());
 }
 
 // 22.1.3.27 String.prototype.toString ( ), https://tc39.es/ecma262/#sec-string.prototype.tostring
diff --git a/Userland/Libraries/LibJS/Tests/builtins/String/String.prototype.toLowerCase.js b/Userland/Libraries/LibJS/Tests/builtins/String/String.prototype.toLowerCase.js
index 4c7cbad50a..da176af6d8 100644
--- a/Userland/Libraries/LibJS/Tests/builtins/String/String.prototype.toLowerCase.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/String/String.prototype.toLowerCase.js
@@ -1,6 +1,10 @@
 test("basic functionality", () => {
     expect(String.prototype.toLowerCase).toHaveLength(0);
 
+    expect("ω".toLowerCase()).toBe("ω");
+    expect("Ω".toLowerCase()).toBe("ω");
+    expect("😀".toLowerCase()).toBe("😀");
+
     expect("foo".toLowerCase()).toBe("foo");
     expect("Foo".toLowerCase()).toBe("foo");
     expect("FOO".toLowerCase()).toBe("foo");
diff --git a/Userland/Libraries/LibJS/Tests/builtins/String/String.prototype.toUpperCase.js b/Userland/Libraries/LibJS/Tests/builtins/String/String.prototype.toUpperCase.js
index 03f463c6e1..51526733c8 100644
--- a/Userland/Libraries/LibJS/Tests/builtins/String/String.prototype.toUpperCase.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/String/String.prototype.toUpperCase.js
@@ -1,6 +1,10 @@
 test("basic functionality", () => {
     expect(String.prototype.toUpperCase).toHaveLength(0);
 
+    expect("ω".toUpperCase()).toBe("Ω");
+    expect("Ω".toUpperCase()).toBe("Ω");
+    expect("😀".toUpperCase()).toBe("😀");
+
     expect("foo".toUpperCase()).toBe("FOO");
     expect("Foo".toUpperCase()).toBe("FOO");
     expect("FOO".toUpperCase()).toBe("FOO");