Watertown is a city in Dodge and Jefferson counties in the U.S. state of Wisconsin. Most of the city's population is in Jefferson County. Division Street, several blocks north of downtown, marks the county line. The population of Watertown was 23,861 at the 2010 census. Of this, 15,402 were in Jefferson County, and 8,459 were in Dodge County.Watertown is the largest city in the Watertown-Fort Atkinson micropolitan area, which also includes Johnson Creek and Jefferson.