diff --git a/Userland/Libraries/LibJS/Runtime/Temporal/PlainDateTimePrototype.cpp b/Userland/Libraries/LibJS/Runtime/Temporal/PlainDateTimePrototype.cpp
index 4163d20c90..4eeced82da 100644
--- a/Userland/Libraries/LibJS/Runtime/Temporal/PlainDateTimePrototype.cpp
+++ b/Userland/Libraries/LibJS/Runtime/Temporal/PlainDateTimePrototype.cpp
@@ -42,6 +42,7 @@ void PlainDateTimePrototype::initialize(GlobalObject& global_object)
     define_native_accessor(vm.names.dayOfYear, day_of_year_getter, {}, Attribute::Configurable);
     define_native_accessor(vm.names.weekOfYear, week_of_year_getter, {}, Attribute::Configurable);
     define_native_accessor(vm.names.daysInWeek, days_in_week_getter, {}, Attribute::Configurable);
+    define_native_accessor(vm.names.daysInMonth, days_in_month_getter, {}, Attribute::Configurable);
 
     u8 attr = Attribute::Writable | Attribute::Configurable;
     define_native_function(vm.names.valueOf, value_of, 0, attr);
@@ -281,6 +282,22 @@ JS_DEFINE_NATIVE_FUNCTION(PlainDateTimePrototype::days_in_week_getter)
     return calendar_days_in_week(global_object, calendar, *date_time);
 }
 
+// 5.3.18 get Temporal.PlainDateTime.prototype.daysInMonth, https://tc39.es/proposal-temporal/#sec-get-temporal.plaindatetime.prototype.daysinmonth
+JS_DEFINE_NATIVE_FUNCTION(PlainDateTimePrototype::days_in_month_getter)
+{
+    // 1. Let dateTime be the this value.
+    // 2. Perform ? RequireInternalSlot(dateTime, [[InitializedTemporalDateTime]]).
+    auto* date_time = typed_this(global_object);
+    if (vm.exception())
+        return {};
+
+    // 3. Let calendar be dateTime.[[Calendar]].
+    auto& calendar = date_time->calendar();
+
+    // 4. Return ? CalendarDaysInMonth(calendar, dateTime).
+    return calendar_days_in_month(global_object, calendar, *date_time);
+}
+
 // 5.3.35 Temporal.PlainDateTime.prototype.valueOf ( ), https://tc39.es/proposal-temporal/#sec-temporal.plaindatetime.prototype.valueof
 JS_DEFINE_NATIVE_FUNCTION(PlainDateTimePrototype::value_of)
 {
diff --git a/Userland/Libraries/LibJS/Runtime/Temporal/PlainDateTimePrototype.h b/Userland/Libraries/LibJS/Runtime/Temporal/PlainDateTimePrototype.h
index 7ca3c4648f..84f980e9b4 100644
--- a/Userland/Libraries/LibJS/Runtime/Temporal/PlainDateTimePrototype.h
+++ b/Userland/Libraries/LibJS/Runtime/Temporal/PlainDateTimePrototype.h
@@ -34,6 +34,7 @@ private:
     JS_DECLARE_NATIVE_FUNCTION(day_of_year_getter);
     JS_DECLARE_NATIVE_FUNCTION(week_of_year_getter);
     JS_DECLARE_NATIVE_FUNCTION(days_in_week_getter);
+    JS_DECLARE_NATIVE_FUNCTION(days_in_month_getter);
     JS_DECLARE_NATIVE_FUNCTION(value_of);
     JS_DECLARE_NATIVE_FUNCTION(to_plain_date);
     JS_DECLARE_NATIVE_FUNCTION(get_iso_fields);
diff --git a/Userland/Libraries/LibJS/Tests/builtins/Temporal/PlainDateTime/PlainDateTime.prototype.daysInMonth.js b/Userland/Libraries/LibJS/Tests/builtins/Temporal/PlainDateTime/PlainDateTime.prototype.daysInMonth.js
new file mode 100644
index 0000000000..6d16b4e150
--- /dev/null
+++ b/Userland/Libraries/LibJS/Tests/builtins/Temporal/PlainDateTime/PlainDateTime.prototype.daysInMonth.js
@@ -0,0 +1,14 @@
+describe("correct behavior", () => {
+    test("basic functionality", () => {
+        const plainDateTime = new Temporal.PlainDateTime(2021, 7, 30);
+        expect(plainDateTime.daysInMonth).toBe(31);
+    });
+});
+
+test("errors", () => {
+    test("this value must be a Temporal.PlainDateTime object", () => {
+        expect(() => {
+            Reflect.get(Temporal.PlainDateTime.prototype, "daysInMonth", "foo");
+        }).toThrowWithMessage(TypeError, "Not a Temporal.PlainDateTime");
+    });
+});