Marcus Marius Gratidianus (died 82 BC) was a Roman praetor, and a partisan of the political faction known as the populares, led by his uncle, Gaius Marius, during the civil war between the followers of Marius and Lucius Cornelius Sulla. As praetor, Gratidianus is known for his policy of currency reform during the economic crisis of the 80s.
Although this period of Roman history is marked by the extreme violence and cruelty practiced by partisans on each side, Gratidianus suffered a particularly vicious death during the Sullan proscriptions; in the most sensational accounts, he was tortured and dismembered by Catiline at the tomb of Quintus Lutatius Catulus, in a manner that evoked human sacrifice, and his severed head was carried through the streets of Rome on a pike.
Born Marcus Gratidius, Gratidianus was the son of Marcus Gratidius of Arpinum and Maria, the sister of Gaius Marius. After his father's death, he was adopted by his uncle, Marcus Marius, whose name he then assumed according to Roman custom, becoming Marcus Marius Gratidianus. Gratidianus' aunt married Marcus Tullius Cicero, grandfather of the celebrated orator. Gratidianus was a close friend of his cousin, the young Cicero. He may also have had a particularly pungent relationship with his brother-in-law; there is reason to believe that his sister, Gratidia, was the first wife of Lucius Sergius Catilina, or "Catiline", who was later accused by Cicero of Gratidianus' torture and murder.
Gratidius, his natural father, was a close friend of Marcus Antonius the orator and consul of 99 BC. He was killed circa 102 BC, while serving as a prefect under Antonius in Cilicia. In 92 BC, Antonius deployed his famed oratorical skills in defending his friend's son when Gratidianus was sued by the oyster-breeder and real-estate speculator Sergius Orata in a civil case involving the sale of a property on the Lucrine Lake. Orata was not without his own high-powered speaker, in the person of Lucius Licinius Crassus. Cicero says Orata was trying to force Gratidianus to buy back the property when Orata's business plan for farm-raised oysters fell through, perhaps because of unforeseen complications arising from water rights or fishing rights. Sometime before 91 BC, a claim, probably also a civil suit, was filed against Gratidianus by Gaius Visellius Aculeo, supported again by Crassus. A Lucius Aelius Lamia spoke on behalf of Gratidianus, but the grounds for the suit are unknown.