Alexander MacDonnell, 3rd Earl of Antrim (1615 – 10 December 1699) was a Roman Catholic peer and military commander in Ireland. He was the son of Randal MacDonnell, 1st Earl of Antrim and his wife, Alice O'Neill (Alice was the daughter of Hugh Ó Neill, 2nd Earl of Tyrone). He married, firstly, Lady Elizabeth Annesley, daughter of Arthur Annesley, 1st Earl of Anglesey. She died childless in 1669. He married, secondly, Helena Burke, daughter of Sir John Burke of Derrymaclachtney, after 1672. They had one son, Randal, and one daughter, Mary.