Gaius Galerius Valerius Maximianus (/ɡəˈlɛəriəs/; c. 258 – May 311) was Roman emperor from 305 to 311. During his reign he campaigned, aided by Diocletian, against the Sassanid Empire, sacking their capital Ctesiphon in 299. He also campaigned across the Danube against the Carpi, defeating them in 297 and 300. Although he was a staunch opponent of Christianity, Galerius ended the Diocletianic Persecution when he issued an Edict of Toleration in Serdica in 311.

Porphyry bust of Galerius
Roman emperor
(in the East)
Augustus1 May 305  early May 311
PredecessorDiocletian and Maximian[1]
SuccessorMaximinus Daza and Licinius
Co-rulersConstantius I (305–306)
Severus (306–307)
Constantine I (306–311)
Maxentius (306–311)
Licinius (308–311)
Caesar1 March or 21 May 293[2][3][4][5][6][7]  1 May 305 (under Diocletian)[1]
Bornc. 258[8][9]
Serdica (Sofia, Bulgaria)[10]
Died5 May 311[11] (aged around 53)
SpouseGaleria Valeria[13]:38
Gaius Galerius Valerius Maximianus[1]
MotherRomula (alleged)[13]:37–8
ReligionRoman polytheism