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

A native of Cyrrhus, Syria, he was the son of Gaius Avidius Heliodorus, a noted orator who had become prefect of Egypt. He had a distinguished military career under Marcus Aurelius during the Parthian War, capturing Seleucia and Ctesiphon, 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; the sources also indicate he was encouraged by Marcus's wife Faustina, who was concerned about her husband's ill health, believing him to be on the verge of death, and felt the need for Cassius to act as a protector in this event, since her son Commodus was still young (13). The evidence, including Marcus's own Meditations, supports the idea that Marcus was indeed quite sick, but contrary to the report that reached Cassius, he had not died. Cassius chose to continue his revolt even after he learned that Marcus was still alive, however.

At first, according to Cassius Dio, Marcus, who was on campaign against tribes in the north, tried to keep the rebellion a secret from his soldiers, but after the news had spread among them, he addressed them. In this speech that Dio attributes to Marcus, he laments the disloyalty of "a dearest friend", while at the same time expressing his hope that Cassius would not be killed or commit suicide, so that he could show mercy. The Senate declared Cassius a public enemy.

It is known that Cassius was recognized as emperor by May 3, since a document of that date is recorded as being in the first year of Cassius's reign. The beginning of his rebellion have been in April 175.

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. Clearly Marcus was in a stronger position, with many more legions available to him than were available to Cassius. "After a dream of empire lasting three months and six days", Cassius was murdered by a centurion; his head was sent to Marcus, who refused to see it and ordered it buried.

The events of his life are known from Cassius Dio's Roman History, and the Historia Augusta.

• William Smith (ed) (1870), Dictionary of Greek and Roman Biography and Mythology Vol 1 p. 626
• Anthony Birley, Marcus Aurelius: A Biography