Marcus Fulvius Flaccus was a Roman senator and an ally of the Gracchi. He became an administrator of the agrarian reform in 130 BC, and as a solution to the problem of land division among the allied cities, proposed Roman citizenship for the allies' citizens, thus introducing a question that vexed Roman politics for many years. Elected consul in 125 BC, he was ordered by the Roman Senate to assist Massilia (modern Marseille) against depredations of the Salluvii. He became the first to overcome the transalpine Ligurians in war and returned in 123 BC with a triumph.
Flaccus was appointed to the Agrarian Commission in 129 BC. In 122 BC he became a tribune to assist Gaius Gracchus in implementing an amended version of his policy of citizenship for Italians, making him the only ex-consul to hold the position of tribune.
He went to found a Roman colony, Colonia Junonia, on the ruins of Carthage. When he and Gracchus failed to win re-election in 121 BC, Flaccus led a mass protest on the Aventine Hill, but the consul Lucius Opimius suppressed it brutally, killing Flaccus among many others and resulting in the suicide of Gracchus.
Plutarch describes him as a born agitator.Cicero describes Flaccus as an orator of moderate gifts and comments that his writings reveal him as a student of letters rather than an orator.
Flaccus had at least two sons: the elder son, possibly named Marcus Fulvius Flaccus after him due to Roman naming conventions, were executed along with the senior Flaccus after being discovered hiding in an abandoned bath or workshop; the younger son Quintus Fulvius Flaccus, who served only as a herald for his father and Gracchus, was forced to death, with Lucius Opimius allowing the young boy to choose his own manner of death.