Gaius Trebonius (died 43 BC) was a military commander and politician of the late Roman Republic, a trusted associate of Julius Caesar who later participated in his assassination.

His father was an eques, but had not been a magistrate, and the son was considered a novus homo, one of several in Caesar's circle. He served as quaestor around 60 BC, and was tribune of the plebs in 55, where he lent his name to the Lex Trebonia that gave five-year commands to Pompey and Crassus. Subsequently he spent five years as a legatus to Caesar in Gaul, who reported well of him, and in 49, Trebonius commanded at the siege of Massilia.

Elected praetor in 48, he was sent to Spain in 47 to fight against Pompey's forces, but was unsuccessful.

Caesar appointed him suffect consul in 45, but even then it was said that Trebonius plotted against him, and in the assassination of the following March, Trebonius was the person who kept Mark Antony outside the Senate while Caesar was being stabbed.

Trebonius was proconsul for Asia in 43; while at Smyrna, he was murdered in an act of treachery by Dolabella.