Gnaeus Flavius, (born late 4th century bc), Roman legal writer and politician who made public the technical rules of legal procedure, which had been kept secret by the patricians and the pontifices (advisers to the king, dictator, or emperor) so that they could maintain their advantage over the plebeians. Flavius learned procedure while serving as secretary to the censor and consul Appius Claudius Caecus. About 304 he made his findings public in a work later known as the Jus Flavianum. From this work the Roman people for the first time could learn the legis actiones, or verbal formulas required to maintain legal proceedings, and the dies fasti, or specified days on which proceedings could be instituted.

Flavius’s resulting popularity caused him to be chosen for several public offices, including that of senator. In 304 he was made curule aedile (magistrate of public buildings and works) over the protests of the nobles, who despised him because he had weakened their power and because he was of plebeian birth.