Phocas (Latin: Focas; Ancient Greek: Φωκάς, romanized: Phōkás; 547 – 5 October 610) was Eastern Roman emperor from 602 to 610. Initially a middle-ranking officer in the Roman army, Phocas rose to prominence as a spokesman for dissatisfied soldiers in their disputes with the court of the Emperor Maurice. When the army rebelled in 602, Phocas emerged as the leader of the mutiny. The revolt led to the overthrow and execution of Maurice in November 602.

Phocas deeply mistrusted the uncooperative elite of Constantinople, to whom he was a usurper and a provincial boor. He therefore attempted to base his regime on relatives installed in high military and administrative positions. He immediately faced challenges in domestic and foreign affairs, and responded with little success. He dealt with domestic opposition with increasing ruthlessness that alienated ever wider circles, including some of his own household. The Sasanian Empire launched a massive invasion of the eastern provinces. Finally, the exarch of Africa, Heraclius the Elder, rebelled against Phocas and gained wide support throughout the empire. Phocas attempted to use border troops to crush the rebellion, but this only resulted in allowing invaders to break into the heartlands of the Empire. Heraclius the Elder's son, Heraclius, took Constantinople on 5 October 610, executed Phocas the same day, and declared himself emperor.

Surviving sources are universally extremely hostile to Phocas. He is described as an incompetent tyrant and usurper who brutally purged any real or perceived opposition and left the Empire wide open to foreign aggression. The veracity of these sources is difficult to ascertain since emperors of the Heraclian dynasty who succeeded Phocas had a vested interest in tarnishing his reputation.

Phocas was most likely born in 547, as he was said to be aged 55 when he became emperor. He and his family were likely of Thraco-Roman or Cappadocian origin. The life of Phocas before his usurpation of the Byzantine Empire's throne is obscure, but he was said to have served as a centurion in the army in Thrace under Emperor Maurice.

Late in Maurice's reign, the army sent Phocas to Constantinople as their spokesperson to complain about the conduct of general Comentiolus. As Phocas presented their case, he argued with the emperor, and as a result, had his beard pulled by an outraged patrician.

In autumn 602, the Thracian army rebelled against Maurice, largely due to exhaustion and outrage over orders to continue campaigning north of the Danube in winter, as well as previous cuts in wages. The army initially sent a delegation, which included Phocas, to general Peter and demanded a dismissal within the Roman borders. But when Maurice reiterated his command, the troops made Phocas their leader and started marching to Constantinople. They insisted that Maurice abdicate and offered to proclaim as successors Theodosius, son and co-emperor of Maurice, or his father-in-law Germanus. As Maurice tried to arrest Germanus, riots broke out in Constantinople. That night, on 22 November 602, Maurice and his family fled on a warship to Bithynia, sending Theodosius on a mission to ask for Persian King Khusrow II's help. According to Theophylact Simocatta (c. 630), Germanus initially made an attempt for the throne, but when rebuffed by the Greens, he paid homage to Phocas, who had emerged as the heavy favorite.

On 23 November 602, Phocas was crowned by the patriarch Cyriacus in the church of St John the Baptist at the Hebdomon. He entered Constantinople in a ceremonial procession two days later, on 25 November, while being unanimously acclaimed.

Maurice and his sons were soon captured and executed. There were, however, stories that Theodosius managed to escape to Persia. Maurice's wife Constantina and daughters were put in the monastery of Nea Metanoia. They were executed around 605, when Constantina and Germanus were found conspiring after hearing rumors of Theodosius' survival.