The Fenian Cycle, Fianna Cycle or Finn Cycle is a body of early Irish literature focusing on the exploits of the mythical hero Finn or Fionn mac Cumhaill and his warrior band the Fianna. Sometimes called the Ossianic Cycle after its narrator Oisín, it is one of the four groupings of Irish mythology along with the Mythological Cycle, the Ulster Cycle, and the Kings' Cycles. Timewise, the Fenian cycle is the third, between the Ulster and Kings' cycles. The cycle also contains stories about other famous Fianna members, including Diarmuid, Caílte, Oisín's son Oscar, and Fionn's rival Goll mac Morna.

Fionn mac Cumhaill, often anglicized Finn McCool or MacCool, is a hero in Irish mythology, as well as in later Scottish and Manx folklore. He is the leader of the Fianna bands of young roving hunter-warriors, as well as being a seer and poet. He is said to have a magic thumb that bestows him with great wisdom. He is often depicted hunting with his hounds Bran and Sceólang, and fighting with his spear and sword. The tales of Fionn and his fiann form the Fianna Cycle or Fenian Cycle, much of it narrated by Fionn's son, the poet Oisín.