The Catuvellauni are a tribe or state of southeastern Britain before the Roman conquest.The fortunes of the Catuvellauni and their kings before the conquest can be traced through numismatic evidence and scattered references in classical histories.

They are mentioned by Dio Cassius, who implies they led the resistance against the conquest in CE 43.

They appear as one of the civitates of Roman Britain in Ptolemy's Geography in the 2nd century, occupying modern Hertfordshire, Bedfordshire and southern Cambridgeshire and based around the town of Verlamion (modern St Albans).Their territory is bordered to the north by the Iceni and Corieltauvi, to the east by the Trinovantes, to the west by the Dobunni and Atrebates, and to the south by the Regnenses and Cantiaci.The Catuvellauni may be related to the Catalauni, a people of Belgic Gaul attested in the region of Châlons-en-Champagne.

The name itself is derived from the Old Brythonic catu - wellauni meaning "battle chieftains" or "battle leaders".

This ultimately derives from the Proto-Celtic "catu", battle, and "wali", to lead.