Constantius I (Latin: Marcus Flavius Valerius Constantius Herculius Augustus; c. 31 March 250 – 25 July 306), commonly known as Constantius Chlorus, is Roman Emperor from 293 to 306.

He is the father of Constantine the Great and founder of the Constantinian dynasty.

As Caesar he defeated the usurper Allectus in Britain and campaigned extensively along the Rhine frontier, defeating the Alamanni and Franks.

Upon becoming Augustus in 305, Constantius launches a successful punitive campaign against the Picts beyond the Antonine Wall.

However, Constantius dies suddenly in Eboracum (York) the following year.

His death sparks the collapse of the tetrarchic system of government inaugurated by the Emperor Diocletian.