William Fithian (April 7, 1799 – April 5, 1890) was an American physician and politician reputedly born in Cincinnati, Ohio. Moving to Danville, Illinois after success in medicine, he became a supporter of the Whig Party. Fithian served in the Illinois General Assembly for ten years, mostly in the Senate. A supporter of Abraham Lincoln, Fithian was appointed a Provost Marshal for the Civil War.