Watertown, city, seat of Codington County, northeastern South Dakota, on the Big Sioux River; incorporated 1885. Situated near Lakes Kampeska and Pelican, it is a resort and a commercial and manufacturing center for a large grain- and livestock-producing region. Products include processed food (notably turkeys), agricultural equipment, clothing, and rubber and plastic goods. The community, laid out in 1878, is named for Watertown, New York. Population 15,649 (1980); 17,592 (1990); 20,237 (2000).