Columbia County is a county located in the U.S. state of Wisconsin. As of the 2020 census, the population was 58,490. Its county seat and largest city is Portage. The county was created in 1846 as part of Wisconsin Territory.

Portage is a city in and the county seat of Columbia County, Wisconsin, United States. The population was 10,581 at the 2020 census, making it the largest city in Columbia County. The city is part of the Madison Metropolitan Statistical Area.