Lancaster is a city in and the county seat of Grant County, Wisconsin, United States. The population was 3,907 at the 2020 census.

Grant County is the most southwestern county in the U.S. state of Wisconsin. As of the 2020 census, the population was 51,938. Its county seat is Lancaster and its largest city is Platteville. The county is named after the Grant River, in turn named after a fur trader who lived in the area when Wisconsin was a territory. Grant County comprises the Platteville Micropolitan Statistical Area. It is in the tri-state area of Wisconsin, Illinois, and Iowa, and is crossed by travelers commuting to Madison, Wisconsin, from a number of eastern Iowan cities, and by residents of northern Illinois traveling to the Twin Cities or La Crosse, Wisconsin.