The Watonwan River is a tributary of the Blue Earth River, 113 miles (182 km) long, in southern Minnesota in the United States. Via the Blue Earth and Minnesota rivers, it is part of the watershed of the Mississippi River, draining an area of 878 square miles (2,270 km2) in an agricultural region. The Watonwan drains about a quarter of the Blue Earth River's watershed. The river was the site of the capture of Bob, Cole and Jim Younger (members of the James–Younger Gang) near Madelia in 1876.