Seoul Metropolitan City Route 21 (Korean: 서울특별시도 제21호선) is an urban road located in Seoul, South Korea. With a total length of 24.2 km (15.0 mi), this road starts from the Siheung Intersection in Geumcheon District, Seoul to Yeonsinnae Station in Eunpyeong District. This route is a part of Asian Highway 1.