diff --git a/Userland/Libraries/LibJS/Runtime/CommonPropertyNames.h b/Userland/Libraries/LibJS/Runtime/CommonPropertyNames.h
index 904813d962..1420fa56ca 100644
--- a/Userland/Libraries/LibJS/Runtime/CommonPropertyNames.h
+++ b/Userland/Libraries/LibJS/Runtime/CommonPropertyNames.h
@@ -381,6 +381,7 @@ namespace JS {
     P(toLocaleUpperCase)                     \
     P(toLowerCase)                           \
     P(toPlainDate)                           \
+    P(toPlainDateTime)                       \
     P(toString)                              \
     P(toTemporalInstant)                     \
     P(toTimeString)                          \
diff --git a/Userland/Libraries/LibJS/Runtime/Temporal/PlainTimePrototype.cpp b/Userland/Libraries/LibJS/Runtime/Temporal/PlainTimePrototype.cpp
index b233444e25..f8550d54b5 100644
--- a/Userland/Libraries/LibJS/Runtime/Temporal/PlainTimePrototype.cpp
+++ b/Userland/Libraries/LibJS/Runtime/Temporal/PlainTimePrototype.cpp
@@ -6,6 +6,8 @@
 
 #include <LibJS/Runtime/GlobalObject.h>
 #include <LibJS/Runtime/Temporal/Calendar.h>
+#include <LibJS/Runtime/Temporal/PlainDate.h>
+#include <LibJS/Runtime/Temporal/PlainDateTime.h>
 #include <LibJS/Runtime/Temporal/PlainTime.h>
 #include <LibJS/Runtime/Temporal/PlainTimePrototype.h>
 
@@ -35,6 +37,7 @@ void PlainTimePrototype::initialize(GlobalObject& global_object)
     define_native_accessor(vm.names.nanosecond, nanosecond_getter, {}, Attribute::Configurable);
 
     u8 attr = Attribute::Writable | Attribute::Configurable;
+    define_native_function(vm.names.toPlainDateTime, to_plain_date_time, 1, attr);
     define_native_function(vm.names.valueOf, value_of, 0, attr);
 }
 
@@ -142,6 +145,24 @@ JS_DEFINE_NATIVE_FUNCTION(PlainTimePrototype::nanosecond_getter)
     return Value(temporal_time->iso_nanosecond());
 }
 
+// 4.3.17 Temporal.PlainTime.prototype.toPlainDateTime ( temporalDate )
+JS_DEFINE_NATIVE_FUNCTION(PlainTimePrototype::to_plain_date_time)
+{
+    // 1. Let temporalTime be the this value.
+    // 2. Perform ? RequireInternalSlot(temporalTime, [[InitializedTemporalTime]]).
+    auto* temporal_time = typed_this(global_object);
+    if (vm.exception())
+        return {};
+
+    // 3. Set temporalDate to ? ToTemporalDate(temporalDate).
+    auto* temporal_date = to_temporal_date(global_object, vm.argument(0));
+    if (vm.exception())
+        return {};
+
+    // 4. Return ? CreateTemporalDateTime(temporalDate.[[ISOYear]], temporalDate.[[ISOMonth]], temporalDate.[[ISODay]], temporalTime.[[ISOHour]], temporalTime.[[ISOMinute]], temporalTime.[[ISOSecond]], temporalTime.[[ISOMillisecond]], temporalTime.[[ISOMicrosecond]], temporalTime.[[ISONanosecond]], temporalDate.[[Calendar]]).
+    return create_temporal_date_time(global_object, temporal_date->iso_year(), temporal_date->iso_month(), temporal_date->iso_day(), temporal_time->iso_hour(), temporal_time->iso_minute(), temporal_time->iso_second(), temporal_time->iso_millisecond(), temporal_time->iso_microsecond(), temporal_time->iso_nanosecond(), temporal_date->calendar());
+}
+
 // 4.3.23 Temporal.PlainTime.prototype.valueOf ( ), https://tc39.es/proposal-temporal/#sec-temporal.plaintime.prototype.valueof
 JS_DEFINE_NATIVE_FUNCTION(PlainTimePrototype::value_of)
 {
diff --git a/Userland/Libraries/LibJS/Runtime/Temporal/PlainTimePrototype.h b/Userland/Libraries/LibJS/Runtime/Temporal/PlainTimePrototype.h
index 4c1694a019..a96f285475 100644
--- a/Userland/Libraries/LibJS/Runtime/Temporal/PlainTimePrototype.h
+++ b/Userland/Libraries/LibJS/Runtime/Temporal/PlainTimePrototype.h
@@ -26,6 +26,7 @@ private:
     JS_DECLARE_NATIVE_FUNCTION(millisecond_getter);
     JS_DECLARE_NATIVE_FUNCTION(microsecond_getter);
     JS_DECLARE_NATIVE_FUNCTION(nanosecond_getter);
+    JS_DECLARE_NATIVE_FUNCTION(to_plain_date_time);
     JS_DECLARE_NATIVE_FUNCTION(value_of);
 };
 
diff --git a/Userland/Libraries/LibJS/Tests/builtins/Temporal/PlainTime/PlainTime.prototype.toPlainDateTime.js b/Userland/Libraries/LibJS/Tests/builtins/Temporal/PlainTime/PlainTime.prototype.toPlainDateTime.js
new file mode 100644
index 0000000000..3584caa7d9
--- /dev/null
+++ b/Userland/Libraries/LibJS/Tests/builtins/Temporal/PlainTime/PlainTime.prototype.toPlainDateTime.js
@@ -0,0 +1,20 @@
+describe("correct behavior", () => {
+    test("length is 1", () => {
+        expect(Temporal.PlainTime.prototype.toPlainDateTime).toHaveLength(1);
+    });
+
+    test("basic functionality", () => {
+        const plainDate = new Temporal.PlainDate(2021, 7, 29);
+        const plainTime = new Temporal.PlainTime(23, 45, 12, 1, 2, 3);
+        const plainDateTime = plainTime.toPlainDateTime(plainDate);
+        expect(plainDateTime.year).toBe(2021);
+        expect(plainDateTime.month).toBe(7);
+        expect(plainDateTime.day).toBe(29);
+        expect(plainDateTime.hour).toBe(23);
+        expect(plainDateTime.minute).toBe(45);
+        expect(plainDateTime.second).toBe(12);
+        expect(plainDateTime.millisecond).toBe(1);
+        expect(plainDateTime.microsecond).toBe(2);
+        expect(plainDateTime.nanosecond).toBe(3);
+    });
+});