Marathon County is a county located in the U.S. state of Wisconsin. As of the 2020 census, the population was 138,013. Its county seat is Wausau. It was founded in 1850, created from a portion of Portage County. At that time the county stretched to the northern border with the upper Michigan peninsula. It is named after the battlefield at Marathon, Greece.

Wausau is a city in and the county seat of Marathon County, Wisconsin, United States, along the Wisconsin River. As of the 2020 census, it had a population of 39,994. It is the core city of the Wausau metropolitan area, which includes all of Marathon County and had a population of 138,013 in 2020. The city's suburbs include Schofield, Weston, Mosinee, Maine, Rib Mountain, Kronenwetter, and Rothschild.