Galerius (260-311) was born in Serdica and was initially a herdsman like his father; before going on to join the Roman army; where he served with distinction under Emperors Aurelian and Probus. When the Tetrarchy was established in 293; he was named as one of the junior co-emperors alongside Constantius; marrying Emperor Diocletian's daughter Valeria.

Galerius fought alongside his father-in-law against the resurgent Sassanid Empire; eventually leading to his sacking of the capital Ctesiphon and his capture of the wife and children of the Sassanid king Narseh; with which he was able to negotiate a long-lasting and favourable peace treaty. When Diocletian and Maximian abdicated in 305; Galerius and Constantius became joint emperors; with Galerius conspiring to secure a stronger power base than his co-ruler. His hopes and plans came to naught when Constantius died a year later and his son; Constantine I; ascended to become emperor of the western half of the empire.

Galerius had been a staunch opponent of Christianity; supposedly prodding Diocletian into enacting the Diocletianic Persecution; the largest and most violent official persecution of Christians in the empire's history; by burning down the Imperial Palace and blaming it on Christian saboteurs. His attitude changed in 311 when he enacted the Edict of Toleration; asking for Christians to pray for him as he suffered through a painful and fatal illness. He died six days later.