Brennus is the name of two Celtic chieftains famous in ancient history:

1. In 390 BC, an army of Celts attacked Rome, led by one Brennus, capturing all of the city except for the Capitoline Hill, which was successfully held against them. This Brennus is famous for exclaiming to the Romans, "Vae Victis!" ("woe to the conquered"). According to legend, during a dispute over the accuracy of the weights used to measure the ransom of gold Brennus demanded, he threw his sword upon the scales and uttered the famous quote.

2. In 279 BC, another army of Celts led by one Brennus invaded Macedonia and northern Greece. The following year they crossed the Bosporus and settled in a part of Asia Minor that came to be called Galatia.

Geoffrey of Monmouth also writes in his Historia Regum Britanniae about a personage named Brennius who conquers Rome. He probably created this character from the two Brenni of history.