diff --git a/Userland/Libraries/LibJS/Runtime/Temporal/PlainYearMonthPrototype.cpp b/Userland/Libraries/LibJS/Runtime/Temporal/PlainYearMonthPrototype.cpp
index 1ea02379db..ea4e44ca38 100644
--- a/Userland/Libraries/LibJS/Runtime/Temporal/PlainYearMonthPrototype.cpp
+++ b/Userland/Libraries/LibJS/Runtime/Temporal/PlainYearMonthPrototype.cpp
@@ -34,6 +34,7 @@ void PlainYearMonthPrototype::initialize(GlobalObject& global_object)
     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);
+    define_native_accessor(vm.names.inLeapYear, in_leap_year_getter, {}, Attribute::Configurable);
 }
 
 static PlainYearMonth* typed_this(GlobalObject& global_object)
@@ -158,4 +159,20 @@ JS_DEFINE_NATIVE_FUNCTION(PlainYearMonthPrototype::months_in_year_getter)
     return Value(calendar_months_in_year(global_object, calendar, *year_month));
 }
 
+// 9.3.10 get Temporal.PlainYearMonth.prototype.inLeapYear, https://tc39.es/proposal-temporal/#sec-get-temporal.plainyearmonth.prototype.inleapyear
+JS_DEFINE_NATIVE_FUNCTION(PlainYearMonthPrototype::in_leap_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 ? CalendarInLeapYear(calendar, yearMonth).
+    return Value(calendar_in_leap_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 468a574615..3a256a5081 100644
--- a/Userland/Libraries/LibJS/Runtime/Temporal/PlainYearMonthPrototype.h
+++ b/Userland/Libraries/LibJS/Runtime/Temporal/PlainYearMonthPrototype.h
@@ -26,6 +26,7 @@ private:
     JS_DECLARE_NATIVE_FUNCTION(days_in_year_getter);
     JS_DECLARE_NATIVE_FUNCTION(days_in_month_getter);
     JS_DECLARE_NATIVE_FUNCTION(months_in_year_getter);
+    JS_DECLARE_NATIVE_FUNCTION(in_leap_year_getter);
 };
 
 }
diff --git a/Userland/Libraries/LibJS/Tests/builtins/Temporal/PlainYearMonth/PlainYearMonth.prototype.inLeapYear.js b/Userland/Libraries/LibJS/Tests/builtins/Temporal/PlainYearMonth/PlainYearMonth.prototype.inLeapYear.js
new file mode 100644
index 0000000000..74e8d2b47e
--- /dev/null
+++ b/Userland/Libraries/LibJS/Tests/builtins/Temporal/PlainYearMonth/PlainYearMonth.prototype.inLeapYear.js
@@ -0,0 +1,14 @@
+describe("correct behavior", () => {
+    test("basic functionality", () => {
+        const plainYearMonth = new Temporal.PlainYearMonth(2021, 7);
+        expect(plainYearMonth.inLeapYear).toBe(false);
+    });
+});
+
+describe("errors", () => {
+    test("this value must be a Temporal.PlainYearMonth object", () => {
+        expect(() => {
+            Reflect.get(Temporal.PlainYearMonth.prototype, "inLeapYear", "foo");
+        }).toThrowWithMessage(TypeError, "Not a Temporal.PlainYearMonth");
+    });
+});