21 Explorer is a bus route running between the City Centre Transit Terminal at Square One and Skymark Avenue.


Buses depart stops every 21 to 24 minutes, during weekday rush hours only.


The route runs along the Transitway, Dixie Road, Eglinton Avenue, and Explorer Drive, supplementing service on the 20 - Rathburn, 107 - Malton Express, and 109 - Meadowvale Express. It often has less crowding than these routes.


The area was originally served by the southern half of the 18 - Northwest-Explorer, which was replaced with this route and the 24 - Northwest in October 2012.