Cathbarr O'Donnell (Gaelic:Cathbarr Ó Domhnaill) (died 1608) was a member of the O'Donnell dynasty of Donegal. He was the son of Sir Hugh O'Donnell, the ruler of Tyrconnell during the Elizabethan era. His mother was Sir Hugh's Scottish second wife Iníon Dubh. After his death, Rosa re-married to the Irish soldier Owen Roe O'Neill. Cathbarr's son with Rosa, Hugh O'Donnell became a Captain in the Spanish Army, serving in his stepfather's regiment in Flanders. He was killed in 1625 during the Siege of Breda.