Watertown (population 19,909) is off I-29 near the Minnesota border. It was home to the last and the first governor of South Dakota, Arthur Calville Mellette. He left his house, built in 1883, and all his assets to the people of Watertown. The restored Mellette House is available for touring. The $10 million Redlin Art Center at the intersection of I-29 and US-212 holds a planetarium, and 92 artworks by the acclaimed wildlife and American painter Terry Redlin. Visitors can visit the Bramble Park Zoo, which is home to 400 birds, mammals, and reptiles from all around the world. The Dakota Sioux Casino is open 24 hours a day; it is run by the Sisseton-Wahpeton Sioux Tribe.

Watertown has a small airport and is serviced by buses.