Ontario Highway 21, also called King's Highway 21, is a provincial highway in southwestern Ontario, Canada. It starts near Ontario Highway 402, almost half way between Sarnia and London. It runs north/northeast for and ends in Owen Sound with Highway 6, Highway 10, and Highway 26. The roadway is referred to as the Bluewater Highway because it remains very close to the eastern shoreline of Lake Huron. It was started in 1927 as Highway 3 running across Kent and Lambton Counties. In 1930 it was extended and renamed King's Highway 21. It is one of Ontario's most treacherous roads for winter driving. Highway 21 is frequently closed during winter because of snow blowing off Lake Huron.