Sir William Parsons, 4th Baronet of Birr Castle (6 May 1731 – 1 May 1791) was an Irish politician and baronet.
He was the son of Sir Laurence Parsons, 3rd Baronet and Mary Sprigge. From 1757 until his death in 1797, Parsons served as member of parliament (MP) in the Irish House of Commons for King's County. He was High Sheriff of King's County in 1779.