Who was the only Roman emperor to be captured and held as a prisoner of war?

Valerian, also known as Valerian the Elder, was Roman Emperor from 253 to 260 AD. He was taken captive by Sassanian Persian king Shapur I after the Battle of Edessa, becoming the only Roman Emperor to be captured as a prisoner of war, causing instability in the Empire.

Valerian's first act as emperor was to make his son Gallienus his Caesar and colleague. Early in his reign, affairs in Europe went from bad to worse, and the whole West fell into disorder. In the East, Antioch had fallen into the hands of a Sassanid vassal and Armenia was occupied by Shapur I (Sapor). Valerian and Gallienus split the problems of the empire between them, with the son taking the West, and the father heading East to face the Persian threat.

In 254, 255, and 257, Valerian again became Consul Ordinarius. By 257, he had recovered Antioch and returned the province of Syria to Roman control. The following year, the Goths ravaged Asia Minor. In 259, Valerian moved on to Edessa, but an outbreak of plague killed a critical number of legionaries, weakening the Roman position, and the town was besieged by the Persians. At the beginning of 260, Valerian was decisively defeated in the Battle of Edessa, and he arranged a meeting with Shapur to negotiate a peace settlement. The truce was betrayed by Shapur, who seized Valerian and held him prisoner for the remainder of his life. Valerian's capture was a tremendous defeat for the Romans.