diff --git a/Userland/Libraries/LibJS/Runtime/Temporal/Calendar.cpp b/Userland/Libraries/LibJS/Runtime/Temporal/Calendar.cpp
index fd2f34ffff..064e823f33 100644
--- a/Userland/Libraries/LibJS/Runtime/Temporal/Calendar.cpp
+++ b/Userland/Libraries/LibJS/Runtime/Temporal/Calendar.cpp
@@ -263,6 +263,16 @@ Value calendar_days_in_year(GlobalObject& global_object, Object& calendar, Objec
     return calendar.invoke(vm.names.daysInYear, &date_like);
 }
 
+// 12.1.19 CalendarMonthsInYear ( calendar, dateLike ), https://tc39.es/proposal-temporal/#sec-temporal-calendarmonthsinyear
+Value calendar_months_in_year(GlobalObject& global_object, Object& calendar, Object& date_like)
+{
+    auto& vm = global_object.vm();
+    // 1. Assert: Type(calendar) is Object.
+
+    // 2. Return ? Invoke(calendar, "monthsInYear", « dateLike »).
+    return calendar.invoke(vm.names.monthsInYear, &date_like);
+}
+
 // 12.1.21 ToTemporalCalendar ( temporalCalendarLike ), https://tc39.es/proposal-temporal/#sec-temporal-totemporalcalendar
 Object* to_temporal_calendar(GlobalObject& global_object, Value temporal_calendar_like)
 {
diff --git a/Userland/Libraries/LibJS/Runtime/Temporal/Calendar.h b/Userland/Libraries/LibJS/Runtime/Temporal/Calendar.h
index 883507cf3a..15f73ebcb7 100644
--- a/Userland/Libraries/LibJS/Runtime/Temporal/Calendar.h
+++ b/Userland/Libraries/LibJS/Runtime/Temporal/Calendar.h
@@ -44,6 +44,7 @@ Value calendar_week_of_year(GlobalObject&, Object& calendar, Object& date_like);
 Value calendar_days_in_week(GlobalObject&, Object& calendar, Object& date_like);
 Value calendar_days_in_month(GlobalObject&, Object& calendar, Object& date_like);
 Value calendar_days_in_year(GlobalObject&, Object& calendar, Object& date_like);
+Value calendar_months_in_year(GlobalObject&, Object& calendar, Object& date_like);
 Object* to_temporal_calendar(GlobalObject&, Value);
 Object* to_temporal_calendar_with_iso_default(GlobalObject&, Value);
 Object* get_temporal_calendar_with_iso_default(GlobalObject&, Object&);
diff --git a/Userland/Libraries/LibJS/Runtime/Temporal/PlainDatePrototype.cpp b/Userland/Libraries/LibJS/Runtime/Temporal/PlainDatePrototype.cpp
index d9daaeb537..944fde1ba3 100644
--- a/Userland/Libraries/LibJS/Runtime/Temporal/PlainDatePrototype.cpp
+++ b/Userland/Libraries/LibJS/Runtime/Temporal/PlainDatePrototype.cpp
@@ -37,6 +37,7 @@ void PlainDatePrototype::initialize(GlobalObject& global_object)
     define_native_accessor(vm.names.daysInWeek, days_in_week_getter, {}, Attribute::Configurable);
     define_native_accessor(vm.names.daysInMonth, days_in_month_getter, {}, Attribute::Configurable);
     define_native_accessor(vm.names.daysInYear, days_in_year_getter, {}, Attribute::Configurable);
+    define_native_accessor(vm.names.monthsInYear, months_in_year_getter, {}, Attribute::Configurable);
 
     u8 attr = Attribute::Writable | Attribute::Configurable;
     define_native_function(vm.names.withCalendar, with_calendar, 1, attr);
@@ -230,6 +231,22 @@ JS_DEFINE_NATIVE_FUNCTION(PlainDatePrototype::days_in_year_getter)
     return Value(calendar_days_in_year(global_object, calendar, *temporal_date));
 }
 
+// 3.3.14 get Temporal.PlainDate.prototype.monthsInYear, https://tc39.es/proposal-temporal/#sec-get-temporal.plaindate.prototype.monthsinyear
+JS_DEFINE_NATIVE_FUNCTION(PlainDatePrototype::months_in_year_getter)
+{
+    // 1. Let temporalDate be the this value.
+    // 2. Perform ? RequireInternalSlot(temporalDate, [[InitializedTemporalDate]]).
+    auto* temporal_date = typed_this(global_object);
+    if (vm.exception())
+        return {};
+
+    // 3. Let calendar be temporalDate.[[Calendar]].
+    auto& calendar = temporal_date->calendar();
+
+    // 4. Return ? CalendarMonthsInYear(calendar, temporalDate).
+    return Value(calendar_months_in_year(global_object, calendar, *temporal_date));
+}
+
 // 3.3.22 Temporal.PlainDate.prototype.withCalendar ( calendar ), https://tc39.es/proposal-temporal/#sec-temporal.plaindate.prototype.withcalendar
 JS_DEFINE_NATIVE_FUNCTION(PlainDatePrototype::with_calendar)
 {
diff --git a/Userland/Libraries/LibJS/Runtime/Temporal/PlainDatePrototype.h b/Userland/Libraries/LibJS/Runtime/Temporal/PlainDatePrototype.h
index e23ae39934..ae433b0277 100644
--- a/Userland/Libraries/LibJS/Runtime/Temporal/PlainDatePrototype.h
+++ b/Userland/Libraries/LibJS/Runtime/Temporal/PlainDatePrototype.h
@@ -30,6 +30,7 @@ private:
     JS_DECLARE_NATIVE_FUNCTION(days_in_week_getter);
     JS_DECLARE_NATIVE_FUNCTION(days_in_month_getter);
     JS_DECLARE_NATIVE_FUNCTION(days_in_year_getter);
+    JS_DECLARE_NATIVE_FUNCTION(months_in_year_getter);
 
     JS_DECLARE_NATIVE_FUNCTION(with_calendar);
     JS_DECLARE_NATIVE_FUNCTION(equals);
diff --git a/Userland/Libraries/LibJS/Tests/builtins/Temporal/PlainDate/PlainDate.prototype.monthsInYear.js b/Userland/Libraries/LibJS/Tests/builtins/Temporal/PlainDate/PlainDate.prototype.monthsInYear.js
new file mode 100644
index 0000000000..c29c255dd9
--- /dev/null
+++ b/Userland/Libraries/LibJS/Tests/builtins/Temporal/PlainDate/PlainDate.prototype.monthsInYear.js
@@ -0,0 +1,14 @@
+describe("correct behavior", () => {
+    test("basic functionality", () => {
+        const date = new Temporal.PlainDate(2021, 7, 23);
+        expect(date.monthsInYear).toBe(12);
+    });
+});
+
+test("errors", () => {
+    test("this value must be a Temporal.PlainDate object", () => {
+        expect(() => {
+            Reflect.get(Temporal.PlainDate.prototype, "monthsInYear", "foo");
+        }).toThrowWithMessage(TypeError, "Not a Temporal.PlainDate");
+    });
+});