South Dakota Highway 19 (SD 19) is a 86.974-mile (139.971 km) state highway in southeastern South Dakota, United States. It connects the Nebraska state line, south of Vermillion, with the southeastern part of the Madison area, via Viborg, Hurley, Parker, and Humboldt. SD 19 formerly entered Centerville, but was shifted to the south. Its former path was redesignated as SD 19A. Its former southern terminus was at Vermillion, but was extended when a new bridge from Nebraska opened.

Clay County is a county in the U.S. state of South Dakota. As of the 2020 census, the population was 14,967. The county seat is Vermillion, which is also home to the University of South Dakota. The county is named for Henry Clay, American statesman, US Senator from Kentucky, and United States Secretary of State in the 19th century.