Brennus (or Brennos) is the name of two Gaulish chieftains famous in ancient history:
- 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 northern Greece and defeated the assembled Greeks at Thermopylae.
The recurrence of the name Brennus makes it possible that it was a title rather than a proper name. In fact, the suffix -us means that it is almost certainly Romanised (similar to the German Armin, Latinized as Arminius). The name Brennus is thus possibly a title rather than a personal name. The name was seen by pre-modern scholars as identical with the modern Welsh word for "king", brenin. Modern opinion holds that the Welsh brenin is derived from the Celtic *brigantinos, meaning "(someone) pre-eminent, outstanding". Another possibility, congruent with Welsh and Gaulish language and mythic sources is the use of the noun bran "crow" as a personal name. Instances of this are seen with the character of Bran the Blessed in the Mabinogion, as well as in several historical Kings of Leinster.
Examples in different forms of the name are:
- Brinno, whose name was said by Tacitus to be that of "a family of rebels".
- The personage named "Brennius" in Geoffrey of Monmouth's Historia Regum Britanniae who conquers Rome, probably created by that author from the two Brenni of history.
- A possible recollection of Geoffrey's "Brennius" is the "Englishman" called Brennus whom the Duke of Norfolk told the Imperial ambassador Eustace Chapuys about in 1530. In arguing Tudor claims to imperial status, the Duke stated that this man had founded Bristol and conquered Rome.