The Vermillion River is a tributary of the Missouri River, 96 miles (154 km) long, in eastern South Dakota in the United States. The origin of the name Vermillion is obscure, it may be a translation of the native Sioux-language name.
It is formed by the confluence of the East Fork Vermillion River and West Fork Vermillion River. The East Fork, approximately 103 miles (166 km) long, rises in Lake Whitewood in Kingsbury County on the Coteau des Prairies. The West Fork, approximately 108 miles (174 km) long, rises in Miner County. Both forks flow south, roughly parallel, joining east of Parker. The combined river flows south and joins the Missouri east of the James River Highlands and 5 miles (8 km) south of Vermillion. Its tributaries include White Stone Creek and Baptist Creek. The Vermillion River drains about 2,180 square miles (5,600 km2) of the southwestern edge of the Coteau des Prairies. Approximately once per 3.5 years, the Vermillion runs dry.
The Vermillion is a north-south river situated between the Big Sioux River and James River.