Mackenzie King Island is one of the Queen Elizabeth Islands north of Canada. It lies north of Melville Island and south of Borden Island, and like them is divided. Most of the island is in Northwest Territories, while a few of its easternmost extremities lie in Nunavut.

Mackenzie King has an area of 1,949 square miles (5,048 km²). The low-lying island was reached by Vilhjalmur Stefansson in 1915, and later named for William Lyon Mackenzie King.