King's Highway 21, commonly referred to as Highway 21, is a provincially maintained highway in the Canadian province of Ontario that begins at Highway 402 midway between Sarnia and London and ends at Highway 6, Highway 10 and Highway 26 in Owen Sound. The roadway is referred to as the Bluewater Highway because it remains very close to the eastern shoreline of Lake Huron.

Highway 21 was first designated by the Department of Highways (DHO) between Highway 3 and Highway 7 in mid-1927 and extended to Goderich in 1934. A year later, a final extension completed the route to Owen Sound. In 1997 and 1998, the portion of the route south of Highway 402 was transferred to the counties in which it laid.

Highway 21 is often subject to winter closures due to lake effect caused by snowsquall, which can create sudden whiteout conditions along the Lake Huron shoreline. Several Emergency Detour Routes have been established further inland to guide drivers around such closures. Care should be taken during the winter months, as these storms can progress rapidly and unexpectedly.

en.wikipedia.org/wiki/Ontario_Highway_21