diff --git a/Libraries/LibJS/Runtime/StringPrototype.cpp b/Libraries/LibJS/Runtime/StringPrototype.cpp
index 73cfd2f921f..c864652c4fa 100644
--- a/Libraries/LibJS/Runtime/StringPrototype.cpp
+++ b/Libraries/LibJS/Runtime/StringPrototype.cpp
@@ -44,6 +44,7 @@ StringPrototype::StringPrototype()
     put_native_function("repeat", repeat, 1);
     put_native_function("startsWith", starts_with, 1);
     put_native_function("indexOf", index_of, 1);
+    put_native_function("toLowerCase", to_lowercase, 0);
 }
 
 StringPrototype::~StringPrototype()
@@ -133,6 +134,26 @@ Value StringPrototype::index_of(Interpreter& interpreter)
     return Value((i32)(ptr - haystack.characters()));
 }
 
+static StringObject* string_object_from(Interpreter& interpreter)
+{
+    auto* this_object = interpreter.this_value().to_object(interpreter.heap());
+    if (!this_object)
+        return nullptr;
+    if (!this_object->is_string_object()) {
+        interpreter.throw_exception<Error>("TypeError", "Not a String object");
+        return nullptr;
+    }
+    return static_cast<StringObject*>(this_object);
+}
+
+Value StringPrototype::to_lowercase(Interpreter& interpreter)
+{
+    auto* string_object = string_object_from(interpreter);
+    if (!string_object)
+        return {};
+    return js_string(interpreter, string_object->primitive_string()->string().to_lowercase());
+}
+
 Value StringPrototype::length_getter(Interpreter& interpreter)
 {
     auto* this_object = interpreter.this_value().to_object(interpreter.heap());
diff --git a/Libraries/LibJS/Runtime/StringPrototype.h b/Libraries/LibJS/Runtime/StringPrototype.h
index 3a2f749aa25..979530c5ddc 100644
--- a/Libraries/LibJS/Runtime/StringPrototype.h
+++ b/Libraries/LibJS/Runtime/StringPrototype.h
@@ -42,6 +42,7 @@ private:
     static Value repeat(Interpreter&);
     static Value starts_with(Interpreter&);
     static Value index_of(Interpreter&);
+    static Value to_lowercase(Interpreter&);
 
     static Value length_getter(Interpreter&);
 };
diff --git a/Libraries/LibJS/Tests/String.prototype.toLowerCase.js b/Libraries/LibJS/Tests/String.prototype.toLowerCase.js
new file mode 100644
index 00000000000..3ebc601e5ae
--- /dev/null
+++ b/Libraries/LibJS/Tests/String.prototype.toLowerCase.js
@@ -0,0 +1,11 @@
+try {
+    assert("foo".toLowerCase() === "foo");
+    assert("Foo".toLowerCase() === "foo");
+    assert("FOO".toLowerCase() === "foo");
+
+    assert(('b' + 'a' + + 'a' + 'a').toLowerCase() === "banana");
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}