For other Romans named "Cassius", see Cassia (gens).

Gaius Cassius Longinus was the prime mover in the conspiracy against Julius Caesar.

Little is known of his early life. In 53 BC he served in the Parthian campaign under M. Licinius Crassus, saved the remnants of the army after the defeat at Carrhae, and for two years successfully repelled the enemy. In 49 BC he became tribune of the plebs. The outbreak of the civil war saved him from being brought to trial for extortion in Syria.

He at first sided with Pompey, and as commander of part of his fleet rendered considerable service in the Mediterranean. After the Battle of Pharsalus, he became reconciled to Caesar, who made him one of his legates. He then became Praetor Peregrinus with the promise of the Syrian province for the ensuing year. The appointment of his junior, Marcus Junius Brutus, as praetor urbanus deeply offended him, and he was one of the busiest Conspirators against Caesar, taking an active part in the actual assassination.

He then left Italy for Syria, raised a considerable army, and defeated P. Cornelius Dolabella, to whom the province had been assigned by the senate. On the formation of the triumvirate, Brutus and he, with their combined armies, crossed the Hellespont, marched through Thrace, and encamped near Philippi in Macedon. Their intention was to starve out the enemy, but they were forced into an engagement. Brutus was successful against Octavian, but Cassius, defeated by Mark Antony, gave up all for lost, and ordered his freedman Pindarus to slay him.

He was lamented by Brutus as "the Last of the Romans", and buried at Thasos. A man of considerable ability, he was a good soldier, and took an interest in literature, but in politics he was actuated by vanity and ambition. His portrait in Shakespeare's Julius Caesar, though vivid, is scarcely historical.

Cassius is also mentioned in Dante's Divina Comedia. In the lowest level of hell is he forever judged to be chewed by Lucifer in one of his three gaping maws. In the other two are Brutus and Judas Iscariot, in the center mouth. These three are therefore seen as the biggest traitors in history. Dante speaks of Cassius as having "sinewy arms", though it is possible he is thinking of another Cassius, as the one of Shakespeare's play is described as having a "lean and hungry look" the marked man distrusts.