Marshfield is a city in Marathon and Wood Counties in the U.S. state of Wisconsin. It is the largest city in Wood County. The small portion of the city that extends into Marathon County is part of the Wausau Metropolitan Statistical Area. The population was 18,800 at the 2000 census. Marshfield is located at the intersection of U.S. Highway 10, Highway 13 and Highway 97. The city forms one of the core areas (the other is Wisconsin Rapids) of the United States Census Bureau's Marshfield-Wisconsin Rapids Micropolitan Statistical Area, which includes all of Wood County . Marshfield is home to the Marshfield Clinic, Saint Joseph's Hospital and the Central Wisconsin State Fair, as well as the World's Largest Round Barn.