King William's College is an independent school for pupils aged 3 to 18, located near Castletown on the Isle of Man. It is a member of the International Baccalaureate and Headmasters' and Headmistresses' Conference organisations. The College operates at two sites in or near Castletown: a main senior school campus on the shore of Castletown Bay, and a prep school in the Westhill part of Castletown, two miles from the main campus. The College was originally for boys only, but became co-educational in the 1980s. It has roughly five hundred pupils, many from beyond the British Isles.

Castletown is a town in the Isle of Man, geographically within the historical parish of Malew but administered separately. Lying at the south of the island, it was the Manx capital until 1869. The centre of town is dominated by Castle Rushen, a well-preserved medieval castle, originally built for a Viking king.