- Brennus (4th century BC), chieftain of the Senones, a Gallic tribe originating from the modern areas of France known as Seine-et-Marne, Loiret, and Yonne; in 387 BC, in the Battle of the Allia, he led an army of Cisalpine Gauls in their attack on Rome.
- Brennus (3rd century BC), one of the leaders of the army of Gauls who invaded Macedon and central Greece and defeated the assembled Greeks at Thermopylae.
The linguistic origins of the name are unclear, despite two different theories linking it to Welsh words. Brennus is not likely to share a common origin with the Welsh personal name Bran (or Brân) meaning 'crow', despite suggestions by scholars as early as 12th Century, when Geoffrey of Monmouth (in his Historia Regum Britanniae) suggested that Brennus was connected to Bran. The similarity of the names is superficial. Recurrence of the name Brennus makes it possible that it was a title rather than a proper name. However, Brennus and the modern Welsh word for "king", brenin (earlier breenhin) are not related, despite assertions by some 19th-century scholars. Brenin is instead derived from the Celtic *brigantinos, meaning '(someone) pre-eminent, outstanding'.
Examples of the name Brennus and variants of it may include:
- Brinno, chief of the Cananefates – a tribe from Germania Inferior – whose name was said by Tacitus to be that of "a family of rebels".
- "Brennius", brother of a legendary king of the Britons Bellinus in Geoffrey's Historia; the two are said to have conquered Gaul and then Rome. Probably created by that author from the two historical Gauls name Brennus. (Geoffrey's Brennius was mentioned by the Duke of Norfolk in 1530, when the duke, while arguing for Tudor claims to imperial status, told Imperial ambassador Eustace Chapuys that Brennius had founded Bristol and conquered Rome. )
The name was given to the French battleship Brennus (commissioned in 1896).
- Guest, Edwin; Origines Celticae (1883)
- Karl, Raimund Thoughts on the Evolution of Celtic Societies University of Wales, 2007, Brennus
- Thomas Healy, Times Literary Supplement 24 June 2005, p. 25; reviewing Philip Schwyzer, Literature, Nationalism, and Memory in Early Modern England and Wales, Cambridge U. Pr., 2005
- John T. Koch, "Brân, Brennos: an instance of Early Gallo-Brittonic history and mythology'", Cambridge Medieval Celtic Studies 20 (Winter 1990:1-20)