Gaius Flavius Fimbria (died 84 BC) was a Roman politician and a violent partisan of Gaius Marius. He fought in the First Mithridatic War.
Fimbria was the son of the Gaius Flavius Fimbria who was consul in 104 BC along with Marius. In 87 BC, the son as either a military tribune or praefectus equitum commanded the cavalry troop that killed the elder son of P. Licinius Crassus, consul in 97 BC and father of the future triumvir. Crassus then committed suicide. Fimbria may also have put to death some members of the Julian family.
Fimbria was sent to the province of Asia in 86 BC as legate to Lucius Valerius Flaccus, but quarrelled with him and was dismissed. Taking advantage of the absence of Flaccus at Chalcedon and the discontent aroused by his avarice and severity, Fimbria stirred up a revolt and killed Flaccus at Nicomedia. He then assumed the command of the army and obtained several successes against Mithridates VI, whom he shut up in Pitane on the coast of Aeolis, and would undoubtedly have captured him had Lucullus co-operated with the fleet.
Fimbria treated most cruelly all the people of Asia who had revolted from Rome or sided with Sulla. Having gained admission to Ilium by declaring that, as a Roman, he was friendly, he massacred the inhabitants and burnt the place to the ground. But in 84 Sulla crossed over from Greece to Asia, made peace with Mithridates, and turned his arms against Fimbria, who, seeing that there was no chance of escape, committed suicide. His troops were made to serve in Asia till the end of the Third Mithridatic War, but two of his officers, Lucius Magius and Lucius Fannius, fled to Mithridates and were of long service to him.