Cassius McDonald Barnes (August 25, 1845 – February 18, 1925) was a soldier in the Union Army in the American Civil War and a lawyer and Republican politician who served as the fourth governor of Oklahoma Territory.

He served for the duration of the war and spent a portion of his enlistment as the secretary to Brigadier General Nathaniel Lyon.

Barnes left the army at the age of twenty and moved to the capital city of Little Rock, Arkansas, where on June 6, 1868, he wed the former Elizabeth Mary Bartlett of North Adams, Massachusetts.

During his four-year term, Barnes defeated the attempts of the 6th Legislature to create numerous additional territorial institutions justified by the growing idea for the formation of the State of Oklahoma.

Governor Barnes continued to live in Guthrie, at which he served as the president of the Logan County Bank.

He and second wife, Rebecca, relocated to Leavenworth, Kansas, where she served as an instructor in a girls seminary and became a postal telegraph operator there.