Isaac Welsh (July 20, 1811 – November 25, 1875) was a Republican politician in the state of Ohio who was in the Ohio House of Representatives, Ohio Senate, and was Ohio State Treasurer from 1872 until his death in 1875, when he was replaced by his son. Isaac Welsh was born July 20, 1811 in Belmont County, Ohio. He was raised on a farm, and had little formal education. As an adult, he moved to Beallsville in Monroe County, and was in merchandising, but soon returned to Belmont. He was a Whig until that party collapsed. Welsh was married to Mary A. Armstrong of Belmont County.