The Iceni ( eye-SEEN-eye, Classical Latin: [ɪˈkeːniː]) or Eceni were a Brittonic tribe of eastern Britain during the Iron Age and early Roman era. Their territory included present-day Norfolk and parts of Suffolk and Cambridgeshire, and bordered the area of the Corieltauvi to the west, and the Catuvellauni and Trinovantes to the south. In the Roman period, their capital was Venta Icenorum at modern-day Caistor St Edmund.

Julius Caesar does not mention the Iceni in his account of his invasions of Britain in 55 and 54 BC, though they may be related to the Cenimagni, whom Caesar notes as living north of the River Thames at that time. The Iceni were a significant power in eastern Britain during Claudius' conquest of Britain in AD 43, in which they allied with Rome. Increasing Roman influence on their affairs led to revolt in AD 47, though they remained nominally independent under king Prasutagus until his death around AD 60. Roman encroachment after Prasutagus' death led his wife Boudica to launch a major revolt from 60–61. Boudica's uprising seriously endangered Roman rule in Britain and resulted in the burning of Londinium and other cities. The Romans finally crushed the rebellion, and the Iceni were increasingly incorporated into the Roman province.