Gaius Avidius Cassius (ca. 130 - July 175) was a Roman usurper who briefly ruled Egypt and Syria in 175.

He was the son of Gaius Avidius Heliodorus . He had a distinguished military career and managed to enter the Senate. He became governor of Syria in 166 and suppressed a revolt of the Bucoli in Egypt that had broken out in 172. In 175, he was proclaimed Roman emperor after the premature news of the death of Marcus Aurelius. Continuing in his revolt even after he learned that Marcus was still alive, the latter began to prepare for war, while at the same time expressing his hope that Cassius would not be killed or commit suicide, so that he would have the opportunity to mercifully pardon the rebellious general.

Although he seized control of some of the most vital parts of the Roman east — Egypt being an important source of grain for the city of Rome — Cassius failed to find widespread support for his rebellion. The governor of Cappadocia, Martius Verus , remained loyal to Marcus Aurelius. After three months, Cassius was murdered by a centurion; his head was sent to Marcus, who refused to see it and ordered it buried.