diff --git a/Userland/Libraries/LibJS/Runtime/Temporal/PlainYearMonthPrototype.cpp b/Userland/Libraries/LibJS/Runtime/Temporal/PlainYearMonthPrototype.cpp
index 053786ef74e..1ea02379db1 100644
--- a/Userland/Libraries/LibJS/Runtime/Temporal/PlainYearMonthPrototype.cpp
+++ b/Userland/Libraries/LibJS/Runtime/Temporal/PlainYearMonthPrototype.cpp
@@ -33,6 +33,7 @@ void PlainYearMonthPrototype::initialize(GlobalObject& global_object)
     define_native_accessor(vm.names.monthCode, month_code_getter, {}, Attribute::Configurable);
     define_native_accessor(vm.names.daysInYear, days_in_year_getter, {}, Attribute::Configurable);
     define_native_accessor(vm.names.daysInMonth, days_in_month_getter, {}, Attribute::Configurable);
+    define_native_accessor(vm.names.monthsInYear, months_in_year_getter, {}, Attribute::Configurable);
 }
 
 static PlainYearMonth* typed_this(GlobalObject& global_object)
@@ -141,4 +142,20 @@ JS_DEFINE_NATIVE_FUNCTION(PlainYearMonthPrototype::days_in_month_getter)
     return Value(calendar_days_in_month(global_object, calendar, *year_month));
 }
 
+// 9.3.9 get Temporal.PlainYearMonth.prototype.monthsInYear, https://tc39.es/proposal-temporal/#sec-get-temporal.plainyearmonth.prototype.monthsinyear
+JS_DEFINE_NATIVE_FUNCTION(PlainYearMonthPrototype::months_in_year_getter)
+{
+    // 1. Let yearMonth be the this value.
+    // 2. Perform ? RequireInternalSlot(yearMonth, [[InitializedTemporalYearMonth]]).
+    auto* year_month = typed_this(global_object);
+    if (vm.exception())
+        return {};
+
+    // 3. Let calendar be yearMonth.[[Calendar]].
+    auto& calendar = year_month->calendar();
+
+    // 4. Return ? CalendarMonthsInYear(calendar, yearMonth).
+    return Value(calendar_months_in_year(global_object, calendar, *year_month));
+}
+
 }
diff --git a/Userland/Libraries/LibJS/Runtime/Temporal/PlainYearMonthPrototype.h b/Userland/Libraries/LibJS/Runtime/Temporal/PlainYearMonthPrototype.h
index 9a6735d4375..468a5746155 100644
--- a/Userland/Libraries/LibJS/Runtime/Temporal/PlainYearMonthPrototype.h
+++ b/Userland/Libraries/LibJS/Runtime/Temporal/PlainYearMonthPrototype.h
@@ -25,6 +25,7 @@ private:
     JS_DECLARE_NATIVE_FUNCTION(month_code_getter);
     JS_DECLARE_NATIVE_FUNCTION(days_in_year_getter);
     JS_DECLARE_NATIVE_FUNCTION(days_in_month_getter);
+    JS_DECLARE_NATIVE_FUNCTION(months_in_year_getter);
 };
 
 }
diff --git a/Userland/Libraries/LibJS/Tests/builtins/Temporal/PlainYearMonth/PlainYearMonth.prototype.monthsInYear.js b/Userland/Libraries/LibJS/Tests/builtins/Temporal/PlainYearMonth/PlainYearMonth.prototype.monthsInYear.js
new file mode 100644
index 00000000000..8e995808cf2
--- /dev/null
+++ b/Userland/Libraries/LibJS/Tests/builtins/Temporal/PlainYearMonth/PlainYearMonth.prototype.monthsInYear.js
@@ -0,0 +1,14 @@
+describe("correct behavior", () => {
+    test("basic functionality", () => {
+        const plainYearMonth = new Temporal.PlainYearMonth(2021, 7);
+        expect(plainYearMonth.monthsInYear).toBe(12);
+    });
+});
+
+describe("errors", () => {
+    test("this value must be a Temporal.PlainYearMonth object", () => {
+        expect(() => {
+            Reflect.get(Temporal.PlainYearMonth.prototype, "monthsInYear", "foo");
+        }).toThrowWithMessage(TypeError, "Not a Temporal.PlainYearMonth");
+    });
+});