Isaac Munroe St. John (November 19, 1827 – April 7, 1880) was a Confederate States Army brigadier general during the American Civil War. He was a lawyer, newspaper editor and civil engineer before the Civil War and a civil engineer after the Civil War. As a civil engineer, he worked for the Baltimore and Ohio Railroad Company (mainly in Maryland) and the Blue Ridge Railroad Company in South Carolina before the Civil War. After the war, he worked for the Louisville, Cincinnati, and Lexington Railroad in Kentucky; the city of Louisville, Kentucky; and the Chesapeake and Ohio Railway Company (mainly in Virginia and West Virginia) after the Civil War.