Procopius (c.325 - May 27, 366), was the cousin on his mother's side of Julian the Apostate, and offered himself as Roman Emperor.

According to Ammianus Marcellinus, he was a native of Cilicia. He took part in the emperor Julian's campaign against the Persian Empire in 363. At the time of Julian's death, there were rumors that he had intended Procopius to be his successor, but when Jovian was elected emperor by the Roman army, Procopius went into hiding to preserve his life. The ancient historians differ on the exact details of Procopius' life in hiding, but agree that he returned to public knowledge at Chalcedon before the house of the senator Strategius suffering from starvation and ignorant of current affairs.

By that time, Jovianus was dead, and Valentinian I shared the purple with his brother Valens. Procopius immediately moved to declare himself emperor. He bribed two legions that were resting at Constantinople to support his efforts, and took control of the imperial city. Shortly after this he proclaimed himself Emperor on September 28, 365, and quickly took control of the provinces of Thrace, and later Bithynia.

Valens was left with the task of dealing with this rebel, and over the next months struggled with both cities and units that wavered in their allegiance. Eventually their armies met at Thyatria , and Procopius' forces were defeated. He fled the battlefield, but was betrayed to Valens by two of his remaining followers. Valens had all three executed May 27, 366.