Vermillion is a city and the county seat of Clay County. It is in the southeastern corner of South Dakota, United States, and is the state's 12th-most populous city. According to the 2020 Census, the population was 11,695. The city lies atop a bluff near the Missouri River. The area has been home to Native American tribes for centuries. French fur traders first visited in the late 18th century. Vermillion was founded in 1859 and incorporated in 1873.