Waukesha County is a county in the U.S. state of Wisconsin. As of the 2020 United States Census, the population was 406,978, making it the third-most populous county in Wisconsin. Its county seat and largest city is Waukesha.

Waukesha is the county seat of Waukesha County, Wisconsin, United States, along the Fox River. Its population was 71,158 at the 2020 census, making it the seventh-most populous city in Wisconsin. It is part of the Milwaukee metropolitan area. The city is adjacent to the Village of Waukesha.