Marcus Flavius Valerius Constantius (c. 250 – 25 July 306), also called Constantius I, was a Roman emperor from 305 to 306—and was father of Constantine the Great, the first Christian emperor of Rome. He was one of the four original members of the Tetrarchy established by Diocletian, first serving as caesar from 293 to 305 and then ruling as augustus until his death. The nickname "Chlorus" (Ancient Greek: Χλωρός, lit. 'the Pale') was first popularized by Byzantine-era historians and not used during the emperor's lifetime.
As a soldier of humble origin, Constantius had a distinguished military career and rose to the top ranks of the army. Around 289, he divorced Helena, the mother of Constantine the Great, to marry a daughter of Emperor Maximian, and in 293 was added to the imperial college by Maximian's colleague Diocletian. Assigned to rule Gaul, Constantius defeated the usurper Carausius there and his successor Allectus in Britain, and campaigned extensively along the Rhine frontier, defeating the Alamanni and Franks. When the Diocletianic Persecution was announced in 303, Constantius ordered the demolition of churches but did not actively hunt down Christians in his domain.  Upon becoming senior emperor in May 305, Constantius launched a successful punitive campaign against the Picts beyond the Antonine Wall. He died suddenly at Eboracum (York) in July the following year.