William Barnes Sr. (May 25, 1824 - February 22, 1913) was an American attorney, author and government official from Albany, New York. He was an anti-slavery activist and a founder of the Republican Party. Barnes served as New York's first state Superintendent of Insurance, and held the office from 1860 to 1870. The works he authored included 1904's Semi-centennial of the Republican Party. He was the son-in-law of Thurlow Weed and the father of Catherine Weed Barnes and William Barnes Jr.