Grant County is a county located in the U.S. state of Wisconsin. As of 2000, the population was 49,597. Its county seat is Lancaster. Estimates for 2008 show a population of 49,238. Grant County also is in the tri-state area with Illinois, Iowa, and Wisconsin, and is crossed by travelers commuting to Madison from a number of eastern Iowan cities, and by residents of northern Illinois traveling to the Twin Cities or La Crosse, Wisconsin.