William McFadden Orr, FRS (2 May 1866 – 14 August 1934) was a British and Irish mathematician. He was born in Comber, County Down and educated at Methodist College Belfast and Queen's College, Belfast under John Purser, before entering St John's College, Cambridge and graduating as Senior Wrangler in 1888. He was elected a fellow of his college, and became a Fellow of the Royal Society in 1909.