Brennius was a legendary king of Northumbria, Scotland, and the Allobroges, as recounted in Geoffrey of Monmouth's Historia Regum Britanniae (c. 1136).

[2] In an effort to win the crown of Britain, Brennius and Belinus waged war between each other to determine who should succeed their father.

After a year of warfare, the joint army managed to subject all the Frankish kingdoms in Gaul to their authority.

Outside Rome, the two consuls, Gabias and Porsenna, sued for peace and offered wealth, tribute, and hostages as a sign of their submission.

Brennius and Belinus continued forward until the walls were breached and the Britons and Gauls invaded the city.

The later is a namesake of Lars Porsena, a King of the Etruscan civilization who is believed to have fought against the recently founded Roman Republic in the decade of the 500s BC.

Brennius was mentioned by Thomas Howard, 3rd Duke of Norfolk, in 1530, when the duke, while arguing in favor of the claims of the House of Tudor claims to imperial status, told Holy Roman Imperial ambassador Eustace Chapuys that Brennius had founded Bristol and conquered Rome.

[3] His ghost appears (as Brennus) in the Jacobean play Fuimus Troes (c. 1607–1625) where he inspires the Britons' defence against Julius Caesar's invasion.