Washington Island is an island of the state of Wisconsin situated in Lake Michigan. Lying about 7 miles (11 km) northeast of the tip of the Door Peninsula, it is part of Door County, Wisconsin. The island has a year-round population of 708 people according to the 2010 census. It has a land area of 60.9 km2 and comprises over 92 percent of the land area of the town of Washington, as well as all of its population. The unincorporated community of Detroit Harbor is situated on the island. It is the largest in a group of islands that includes Plum, Detroit, Hog, Pilot, Fish, and Rock Islands. These islands form the Town of Washington. Detroit Harbor bay is on the south side of the island. A large part of Washington Island's economy is based on tourism.

Lake Michigan is one of the five Great Lakes of North America. It is the second-largest of the Great Lakes by volume and the third-largest by surface area, after Lake Superior and Lake Huron. To the east, its basin is conjoined with that of Lake Huron through the 3+1⁄2-mile (5.6-kilometer) wide, 295-foot deep Straits of Mackinac, giving it the same surface elevation as its easterly counterpart; geologically, the two bodies are a single lake that is, by area, the largest freshwater lake in the world.