Gaius Flavius Valerius Constantius (March 31, 250–July 25, 306) was an emperor of the Western Roman Empire (305–306). He was commonly called Chlorus (the Pale), an epithet given to him by Byzantine historians. He was the father of Constantine the Great.

Constantius was of Illyrian origin. Historia Augusta reports him being son of Eutropius and Claudia, daughter of Crispus. Crispus was reportedly a brother to Roman Emperors Claudius II and Quintillus. However, historians suspect this connection to be a genealogical fabrication created by his son Constantine, thus connecting his family to two rather highly regarded predecessors.

The early years of Constantius' life are sketchy. Geoffrey of Monmouth writes in his Historia Regum Britanniae that Constantius was a Roman senator early in his career. The text states that early in his career, he forced Spain to submit to Roman rule and that he continued throughout his life to increase the power of Rome. Probably around 296, Constantius was sent to Britain to quell the rebellions caused by various tribes. This account comes at odds with the story of King Cole, who was said to have stopped the rebellions before submitting to Rome. It is possible that their two reigns overlapped somewhat, since Cole gave Rome control over the island.

Nevertheless, Constantius made peace with the Britons and ruled them throughout the remainder of his life. At this point, a contradiction between the origin of Helena comes into play. Most sources cite her as the daughter of an innkeeper while Monmouth and the Welsh chronicles state she was Coel's daughter. Regardless, this Helen gave birth to Constantine.

Having distinguished himself by his military ability and his able and gentle rule of Dalmatia, he was, on March 1, 293, adopted and appointed Caesar by Maximian, whose step-daughter, Flavia Maximiana Theodora, he had married in 289 after renouncing his wife, Helena. In the distribution of the provinces, Gaul and Britain were allotted to Constantius.

In 298 Constantius overthrew the Alamanni in the territory of the Lingones (Langres) and strengthened the defenses of the Rhine frontier. During the persecution of the Christians in 303, he behaved with great humanity. He obtained the title of Augustus on May 1, 305, and died the following year in Eboracum (York) during an expedition against the Picts and Scots. He was succeeded by his son Constantine in the rule of Britain and Galerius in the rule of Rome.