Cassivellaunus is an historical British chieftain who leads the defense against Julius Caesar's second expedition to Britain in 54 BCE.

The first British person whose name is recorded, Cassivellaunus leads an alliance of tribes against Roman forces, but eventually surrenders after his location is revealed to Caesar by defeated Britons.

Cassivellaunus made an impact on the British consciousness.

He appears in British legend as Cassibelanus, one of Geoffrey of Monmouth's kings of Britain, and in the Mabinogion, the Brut y Brenhinedd and the Welsh Triads as Caswallawn, son of Beli Mawr.

His name in Brythonic *Kađđiwellaunos, from *kađđi- "passion, love, hate" + *welnā- "to lead" + -aunos "one who", translates as either "leader of the Cassi [tribe]", and therefore may be more of a title rather than his actual given name (although the two need not be mutually exclusive), or as "The Passionate Leader".