Watertown is county seat of Codington County, South Dakota, United States. The population was 21,995 at the 2013 census. It is the fifth largest city in South Dakota. 

Watertown is home to the Redlin Art Center which houses many of the original art works of Terry Redlin, one of America's most popular wildlife artists. Watertown also is home to the Bramble Park Zoo and the wildlife artist Joshua Spies and TV personality Benny Spies. 

Watertown was founded in 1879 as a rail terminus when the Chicago & Northwestern Railroad reactivated part of a line it had constructed to Lake Kampeska. The city was named after Watertown, New York, the hometown of brothers John E. Kemp and Oscar P. Kemp, some of the city's founders. 

During the 1880s, Watertown prospered as a transportation hub after the railroads had extended further west. Along with several other cities, Watertown was a candidate as capital of the new state of South Dakota, although it lost out to the more centrally located Pierre. 

In the mid-20th century, Interstate 29 was constructed through eastern South Dakota. The route included a slight bend to bring the highway closer to Watertown. The interstate's construction has been a major economic benefit to the larger communities along its route, including Watertown.