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.

Following the Battle of Magh Slecht in 1256, Bréifne split into West Breifne (ruled by the Ua Ruairc) and East Breifne (ruled by the Ua Raghallaigh).

Bréifne was part of the province of Connacht until the reign of Queen Elizabeth I. In that time it was shired into the modern counties Cavan and Leitrim, Leitrim remaining a part of the province of Connacht while Cavan became part of Ulster.

Breifne is said to derive from an obsolete Irish word meaning "hilly", a description which describes the topography of this part of Ireland. But this derivation is opposed by the likes of O'Connell and MacEoin. It was referred to as the rough third of Connacht. Alternatively, the Metrical Dindshenchas states the name is derived from Brefne, daughter of Beoan mac Bethaig, the grandson of Nemed, a brave soldier-woman. She was slain by Regan after whom Tomregan is supposedly named.

In ancient times the area that became known as Bréifne was said to be occupied by the Erdini, called in Irish 'Ernaigh', who possessed the entire country bordering Lough Erne.

At the time of the Christianization of Ireland (c. 5th–6th century), groups believed to be in or near Breifne included the Glasraighe, Masraige, Dartraige, Armhaighe, Gallraighe, the Fir Manach, and the Gailenga.

Around the 6th century, a people known as the Conmaicne Rein are thought to have moved north from around the present Dunmore in County Galway and settled in Magh Rein (the area around Fenagh). From here they peopled what is now South Leitrim, which became known as Magh Rein, and its inhabitants as the Conmaicne Magh Rein. They consisted of different family groupings – Muintir Eolais, Muintir Cearbhallain (O Mulvey), and Cinel Luachain, among others.