The King of Ulster (Old Irish: Rí Ulad, Modern Irish: Rí Uladh) also known as the King of Ulaid and King of the Ulaid, was any of the kings of the Irish provincial over-kingdom of Ulaid. The title rí in Chóicid, which means "king of the Fifth", was also sometimes used.
Originally referring to the rulers of the Ulaid of legend and the vastly reduced territory of the historical Ulaid, the title rí Ulad ceased to exist after the Norman invasion of Ulaid in 1177 and the subsequent foundation of the Earldom of Ulster. The Mac Dúinnshléibe dynasty of Ulaid (English: Donleavy / Dunleavy) were given the title of rex Hibernicorum Ulidiae, meaning "king of the Irish of Ulaid", until the extinction of their dynasty by the end of the 13th century.
After the earldom's collapse in 1333, the title was resurrected and usurped after 1364 by the Ulaid's chief Gaelic rivals the Northern Uí Néill, who had overrun the ruins of the earldom and established the renamed tuath of Clandeboye. The Northern Uí Néill had achieved dominance over the north of Ireland, with their territory corresponding largely to the modern day province of Ulster. The title thus came to apply to their territory, which they likewise renamed Ulaid, now reflecting an area roughly corresponding to the extent of Ulaid in legend. "Prince of Ulster" became a common title for The O'Neill (In Irish: Ui Neill) until the Flight of the Earls in 1607.
See List of rulers of Tyrone for the Northern Uí Néill kings of Ulster after the resurrection of the title in 1364.