Manitoulin Island (ˌmænəˈtuːlᵻn ) is an island in Lake Huron, located within the borders of the Canadian province of Ontario, in the bioregion known as Laurentia. With an area of , it is the largest lake island in the world, large enough that it has over 100 lakes itself. In addition to the historic Anishinaabe and European settlement of the island, archaeological discoveries at Sheguiandah have demonstrated Paleo-Indian and Archaic cultures dating from 10,000 BC to 2,000 BC.
The current name of the island is the English version, via French, of the Ottawa or Ojibwe name Manidoowaaling (ᒪᓂᑝᐙᓕᓐᒃ), which means "cave of the spirit". It was named for an underwater cave where a powerful spirit is said to live. By the 19th century, the Odawa "l" was pronounced as "n". The same word with a newer pronunciation is used for the town Manitowaning (19th-century Odawa "Manidoowaaning"), which is located on Manitoulin Island near the underwater cave where legend has it that the spirit dwells. The modern Odawa name for Manitoulin Island is Mnidoo Mnis, meaning "Spirit Island".
Manitoulin Island contains a number of lakes of its own. In order of size, its three most prominent lakes are Lake Manitou, Lake Kagawong and Lake Mindemoya. These three lakes in turn have islands within them, the largest of these being Lake Mindemoya's Treasure Island, located in the centre of Mindemoya.
The island is the site of the administrative office of the band government of the Sheshegwaning First Nation.
The island has an area of , making it the largest freshwater island in the world, the 174th largest island in the world and Canada's 31st largest island.
The island separates the larger part of Lake Huron to its south and west from Georgian Bay to its east and the North Channel to the north.
Manitoulin Island itself has 108 freshwater lakes, some of which have their own islands; in turn several of these "islands within islands" have their own ponds. Lake Manitou, at , is the largest lake in a freshwater island in the world, and Treasure Island in Lake Mindemoya is the largest island in a lake on an island in a lake in the world.