Mark Barnett (born September 4, 1954) is an American attorney; the 28th attorney general of South Dakota between 1991 and 2003 and a circuit court judge between 2007 and 2019.
Barnett won the general election by defeating Michael Butler. He won with 150,109 (59.49%) votes while Butler received 102,231 votes (40.51%).
Barnett was re-elected by defeating Democrat Randy Turner and Libertarian Bert Olson. Barnett received 192,147 (62.33%) votes; Randy received 106,709 (34.62%) votes and Bert received 9,410 (3.05%) votes. 
Barnett was elected for a third term as attorney general. He was unopposed for the first time in the history of attorney general elections in South Dakota. 
As of October, 2019, Barnett is the longest-serving attorney general in South Dakota history, with 12 years of service.
Barnett argued two cases before the United States Supreme Court as attorney general, winning both cases.
Barnett argued on behalf of the state, winning a 7–2 decision that the federal Flood Control and Cheyenne River Acts had abrogated the tribe’s right, guaranteed under the Fort Laramie Treaty, to regulate hunting and fishing on their lands by non-Indians.