Lake Michigan (/ˈmɪʃɪɡən/ ^ⓘ MISH-ig-ən) is one of the five Great Lakes of North America. It is the second-largest of the Great Lakes by volume (1,180 cu mi; 4,900 km³) and depth (923 ft; 281 m) after Lake Superior and the third-largest by surface area (22,405 sq mi; 58,030 km²), after Lake Superior and Lake Huron. To the east, its basin is conjoined with that of Lake Huron through the wide and deep Straits of Mackinac, giving it the same surface elevation as its eastern counterpart; hydrologically, the two bodies are a single lake that is, by area, the largest freshwater lake in the world.

Lake Michigan is the only Great Lake located fully in the United States; the other four are shared between the U.S. and Canada. It is the world's largest lake, by area, located fully in one country, and is shared, from west to east, by the U.S. states of Wisconsin, Illinois, Indiana, and Michigan. Ports along its shores include Chicago, Illinois, Gary, Indiana, Milwaukee and Green Bay, Wisconsin, and Muskegon, Michigan. To the north, the lake is flanked by long bays, including Green Bay in the northwest, and Grand Traverse and Little Traverse bays in the northeast. The word michigan is believed to come from the Ojibwe ᒥᓯᑲᒥ (michi-gami or mishigami), meaning "great water".

Some of the most well-studied early human inhabitants of the Lake Michigan region were the Hopewell Native Americans. Their culture declined after 800 AD, when, for the following few hundred years, the region was the home of peoples known as the Late Woodland Native Americans. In the early 17th century, when Western European explorers made their first forays into the region, they encountered descendants of the Late Woodland Native Americans, mainly the historic Ojibwe, Menominee, Noquet, Sauk, Meskwaki, Ho-Chunk, Miami, Odawa and Potawatomi peoples. The French explorer Jean Nicolet is believed to have been the first European to reach Lake Michigan, possibly in 1634 or 1638. In the earliest European maps of the region, the name of Lake Illinois has been found (named for the Illinois Confederation of tribes), in addition to that of "Michigan". During the 1640s and 1650s, the Beaver Wars (over the fur trade with the European colonies), initiated by the Iroquois, forced a massive demographic-shift, as their western neighbors fled the violence. The Iroquois sought refuge to the west and north of Lake Michigan.

The Straits of Mackinac were an important Native American travel corridor and fur-trade route. Located on the southern side of the straits sits the town of Mackinaw City, Michigan, the site of Fort Michilimackinac (a reconstructed French fort founded in 1715); on the northern side is St. Ignace, Michigan, site of a French Catholic mission to the Indians (founded in 1671). In 1673, Jacques Marquette, Louis Jolliet, and their crew of five Métis voyageurs followed Lake Michigan to Green Bay and up the Fox River, nearly to its headwaters, in their search for the Mississippi River. By the late 18th century, the eastern portions of the straits were controlled by Fort Mackinac on Mackinac Island, a British colonial and early American military base and fur trade center, founded in 1781.

With the advent of European exploration into the area in the late 17th century, Lake Michigan became used as part of a line of waterways leading from the Saint Lawrence River to the Mississippi River and thence to the Gulf of Mexico. French coureurs des bois and voyageurs established small ports and trading communities, such as Green Bay, on the lake during the late 17th and early 18th centuries. In the 19th century, Lake Michigan was integral to the development of Chicago and the Midwestern United States west of the lake. For example, 90% of the grain shipped from Chicago traveled by ships east over Lake Michigan during the antebellum years. The volume rarely fell below 50% after the Civil War even with the major expansion of railroad shipping.

The first person to reach the deep bottom of Lake Michigan was J. Val Klump, a scientist at the University of Wisconsin–Milwaukee in 1985. Klump reached the bottom via submersible as part of a research expedition. The warming of Lake Michigan was the subject of a 2018 report by Purdue University. In each decade since 1980, steady increases in obscure surface temperature have occurred. This is likely to lead to decreasing native habitat and to adversely affect native species survival, including game fish.

The Milwaukee Reef, running under Lake Michigan from a point between Milwaukee and Racine to a point between Grand Haven and Muskegon, divides the lake into northern and southern basins. Each basin has a clockwise flow of water, deriving from rivers and winds. Prevailing westerly winds tend to move the surface water toward the east, producing a moderating effect on the climate of western Michigan. There is a mean difference in summer water temperatures of 5 to 10 degrees Fahrenheit (2 to 5 degrees Celsius) between the Wisconsin and Michigan shores.

Hydrologically Michigan and Huron are the same body of water (sometimes called Lake Michigan-Huron) but are normally considered distinct. Counted together, it is the largest body of fresh water in the world by surface area. The Mackinac Bridge is generally considered the dividing line between them. The main inflow to Lake Michigan from Lake Superior, through Lake Huron, is controlled by the locks operated by the bi-national Lake Superior Board of Control.