Wausau is a city in and the county seat of Marathon County, Wisconsin, United States. The Wisconsin River divides the city into east and west. The city is adjacent to the Town of Wausau. As of the 2010 census, Wausau had a population of 39,106. It is the core city of the Wausau Metropolitan Statistical Area (MSA), which includes all of Marathon County and had a population of 134,063 at the 2010 census. The current mayor of Wausau is Robert Mielke.