diff --git a/Userland/Libraries/LibJS/Runtime/Temporal/PlainDatePrototype.cpp b/Userland/Libraries/LibJS/Runtime/Temporal/PlainDatePrototype.cpp
index 7f49778135..7718a85d8e 100644
--- a/Userland/Libraries/LibJS/Runtime/Temporal/PlainDatePrototype.cpp
+++ b/Userland/Libraries/LibJS/Runtime/Temporal/PlainDatePrototype.cpp
@@ -428,6 +428,7 @@ JS_DEFINE_NATIVE_FUNCTION(PlainDatePrototype::to_string)
 }
 
 // 3.3.29 Temporal.PlainDate.prototype.toLocaleString ( [ locales [ , options ] ] ), https://tc39.es/proposal-temporal/#sec-temporal.plaindate.prototype.tolocalestring
+// NOTE: This is the minimum toLocaleString implementation for engines without ECMA-402.
 JS_DEFINE_NATIVE_FUNCTION(PlainDatePrototype::to_locale_string)
 {
     // 1. Let temporalDate be the this value.
diff --git a/Userland/Libraries/LibJS/Runtime/Temporal/PlainYearMonthPrototype.cpp b/Userland/Libraries/LibJS/Runtime/Temporal/PlainYearMonthPrototype.cpp
index f045797438..7404d9a522 100644
--- a/Userland/Libraries/LibJS/Runtime/Temporal/PlainYearMonthPrototype.cpp
+++ b/Userland/Libraries/LibJS/Runtime/Temporal/PlainYearMonthPrototype.cpp
@@ -39,6 +39,7 @@ void PlainYearMonthPrototype::initialize(GlobalObject& global_object)
 
     u8 attr = Attribute::Writable | Attribute::Configurable;
     define_native_function(vm.names.toString, to_string, 0, attr);
+    define_native_function(vm.names.toLocaleString, to_locale_string, 0, attr);
     define_native_function(vm.names.valueOf, value_of, 0, attr);
     define_native_function(vm.names.getISOFields, get_iso_fields, 0, attr);
 }
@@ -208,6 +209,24 @@ JS_DEFINE_NATIVE_FUNCTION(PlainYearMonthPrototype::to_string)
     return js_string(vm, *string);
 }
 
+// 9.3.18 Temporal.PlainYearMonth.prototype.toLocaleString ( [ locales [ , options ] ] ), https://tc39.es/proposal-temporal/#sec-temporal.plainyearmonth.prototype.tolocalestring
+// NOTE: This is the minimum toLocaleString implementation for engines without ECMA-402.
+JS_DEFINE_NATIVE_FUNCTION(PlainYearMonthPrototype::to_locale_string)
+{
+    // 1. Let yearMonth be the this value.
+    // 2. Perform ? RequireInternalSlot(yearMonth, [[InitializedTemporalYearMonth]]).
+    auto* year_month = typed_this(global_object);
+    if (vm.exception())
+        return {};
+
+    // 3. Return ? TemporalYearMonthToString(yearMonth, "auto").
+    auto string = temporal_year_month_to_string(global_object, *year_month, "auto"sv);
+    if (vm.exception())
+        return {};
+
+    return js_string(vm, *string);
+}
+
 // 9.3.20 Temporal.PlainYearMonth.prototype.valueOf ( ), https://tc39.es/proposal-temporal/#sec-temporal.plainyearmonth.prototype.valueof
 JS_DEFINE_NATIVE_FUNCTION(PlainYearMonthPrototype::value_of)
 {
diff --git a/Userland/Libraries/LibJS/Runtime/Temporal/PlainYearMonthPrototype.h b/Userland/Libraries/LibJS/Runtime/Temporal/PlainYearMonthPrototype.h
index 97509e8a86..feb9130be3 100644
--- a/Userland/Libraries/LibJS/Runtime/Temporal/PlainYearMonthPrototype.h
+++ b/Userland/Libraries/LibJS/Runtime/Temporal/PlainYearMonthPrototype.h
@@ -28,6 +28,7 @@ private:
     JS_DECLARE_NATIVE_FUNCTION(months_in_year_getter);
     JS_DECLARE_NATIVE_FUNCTION(in_leap_year_getter);
     JS_DECLARE_NATIVE_FUNCTION(to_string);
+    JS_DECLARE_NATIVE_FUNCTION(to_locale_string);
     JS_DECLARE_NATIVE_FUNCTION(value_of);
     JS_DECLARE_NATIVE_FUNCTION(get_iso_fields);
 };
diff --git a/Userland/Libraries/LibJS/Tests/builtins/Temporal/PlainYearMonth/PlainYearMonth.prototype.toLocaleString.js b/Userland/Libraries/LibJS/Tests/builtins/Temporal/PlainYearMonth/PlainYearMonth.prototype.toLocaleString.js
new file mode 100644
index 0000000000..ec4879c6e1
--- /dev/null
+++ b/Userland/Libraries/LibJS/Tests/builtins/Temporal/PlainYearMonth/PlainYearMonth.prototype.toLocaleString.js
@@ -0,0 +1,23 @@
+describe("correct behavior", () => {
+    test("length is 0", () => {
+        expect(Temporal.PlainYearMonth.prototype.toLocaleString).toHaveLength(0);
+    });
+
+    test("basic functionality", () => {
+        let plainYearMonth;
+
+        plainYearMonth = new Temporal.PlainYearMonth(2021, 7);
+        expect(plainYearMonth.toLocaleString()).toBe("2021-07");
+
+        plainYearMonth = new Temporal.PlainYearMonth(2021, 7, { toString: () => "foo" }, 6);
+        expect(plainYearMonth.toLocaleString()).toBe("2021-07-06[u-ca=foo]");
+    });
+});
+
+describe("errors", () => {
+    test("this value must be a Temporal.PlainYearMonth object", () => {
+        expect(() => {
+            Temporal.PlainYearMonth.prototype.toLocaleString.call("foo");
+        }).toThrowWithMessage(TypeError, "Not a Temporal.PlainYearMonth");
+    });
+});