Publius Licinius Egnatius Gallienus ( c. 218 – September 268) was Roman emperor with his father Valerian from 253 to 260 and alone from 260 to 268. He ruled during the Crisis of the Third Century that nearly caused the collapse of the empire. He won numerous military victories against usurpers and Germanic tribes, but was unable to prevent the secession of important provinces. His 15-year reign was the longest in half a century.

Born into a wealthy and traditional senatorial family, Gallienus was the son of Valerian and Mariniana. Valerian became Emperor in September 253 and had the Roman Senate elevate Gallienus to the rank of Augustus. Valerian divided the empire between him and his son, with Valerian ruling the east and his son the west. Gallienus defeated the usurper Ingenuus in 258 and destroyed an Alemanni army at Mediolanum in 259.

The defeat and capture of Valerian at Edessa in 260 by the Sasanian Empire threw the Roman Empire into the chaos of civil war. Control of the whole empire passed to Gallienus. He defeated the eastern usurpers Macrianus Major and Lucius Mussius Aemilianus in 261–262 but failed to stop the formation of the breakaway Gallic Empire under general Postumus. Aureolus, another usurper, proclaimed himself emperor in Mediolanum in 268 but was defeated outside the city by Gallienus and besieged inside. While the siege was ongoing, Gallienus was assassinated, stabbed to death by the officer Cecropius, as part of a conspiracy.