The Kingdom of Breifne or Bréifne (Irish: [ˈbʲɾʲeːfʲnʲə]), anglicized as Breffny, was a medieval overkingdom in Gaelic Ireland. It comprised what is now County Leitrim, County Cavan and parts of neighbouring counties, and corresponds roughly to the Roman Catholic Diocese of Kilmore. It had emerged by the 10th century, as a confederation of túatha headed by an overking drawn from the Uí Briúin Bréifne.
By the 11th century, Bréifne was ruled by the Ua Ruairc (O'Rourke) dynasty. The kingdom reached the height of its power in the 12th century, under Tigernán Ua Ruairc. During the latter part of his reign, Bréifne took part in campaigns against the Norman invasion of Ireland. His assassination by the Anglo-Normans in 1172 was followed by a succession dispute, and a conflict between the Ua Ruairc and Ua Raghallaigh (O'Reilly) dynasties.