Alger County is a county in the Upper Peninsula of the U.S. state of Michigan. As of the 2020 Census, the population was 8,842. It is the state's second-largest county by area, including the waters of Lake Superior. The county seat is Munising.

The Upper Peninsula of Michigan—also known as Upper Michigan or colloquially the U.P.—is the northern and more elevated of the two major landmasses that make up the U.S. state of Michigan; it is separated from the Lower Peninsula by the Straits of Mackinac. It is bounded primarily by Lake Superior to the north, separated from the Canadian province of Ontario at the east end by the St. Marys River, and flanked by Lake Huron and Lake Michigan along much of its south. Although the peninsula extends as a geographic feature into the state of Wisconsin, the state boundary follows the Montreal and Menominee rivers and a line connecting them.