Marcus Octavius (lived 2nd century BC) was a Roman tribune and a major rival of Tiberius Gracchus. A serious and discreet person, he earned himself a reputation as an influential orator. Though originally close friends, Octavius became alarmed by Gracchus's populist agenda and, at the behest of the Roman senate, repeatedly vetoed Gracchus' programmes of land reform. Gracchus responded by ultimately having the Plebeian Assembly deprive him of his office and eject him from the Assembly's meeting place in 133 BC. This action led to a serious escalation in the confrontation between the traditionalists and the reformers. The action was unprecedented and contravened mos maiorum (Latin term for "the traditional way of doing things") The Assemblies decision to depose Marcus Octavius in order to ensure Tiberius Gracchus' land Bill was passed, revealed the true power that the tribunate and the assembly had which it did not have prior to 133BC. The power of the Assembly and the Tribune was one of the factors that led to the decreasing influence of the Senate in Roman politics which led to the Roman civil wars and ultimately the fall of the Roman Republic.
Our primary source for him is Plutarch's life of Tiberius. According to Plutarch, Gaius Octavius was one of his descendants, thus making him an ancestor of Roman Emperor Augustus.