Amherst Island is located in Lake Ontario, 10 kilometres (6 mi) west of Kingston, Ontario, Canada
The island is part of Loyalist Township in Lennox and Addington County. Amherst Island is located about 3 kilometres (2 mi) offshore from the rest of Loyalist Township and is serviced by an automobile and truck ferry from Millhaven. The island measures over 20 kilometres (12 mi) in length from Bluff Point in the southwest to Amherst Bar in the northeast and over 7 kilometres (4.3 mi) at its widest point across. The island is about 70 square kilometres (27 sq mi) in size and is one of the largest islands in the Great Lakes.