Taiwan Provincial Highway No. 21 is a Provincial Highway in Taiwan that begins at Tianleng (天冷), Dongshi District, Taichung City, at the intersection with Provincial Highway No. 8, and ends at Tataka(塔塔加), where it is connected with Provincial Highway No. 18 in Yushan National Park. It is the primary highway to Sun Moon Lake, a popular tourist destination. The total length is 145 km.