Arbitio was a Roman general (magister militum) and Consul who lived in the middle of the 4th century.
He was a general of Constantine the Great and reached the highest military positions in the Roman army under Constantius II and became commander of the cavalry in Gaul. In 355 he was made Consul together with Quintus Flavius Maesius Egnatius Lollianus.
Arbitio was a well trusted courtier of Constantius and some modern historians have suggested he was his military strongman.
Arbitio intrigued against Silvanus, Ursicinus and Barbatio and played a role their downfall. Historian Ammianus Marcellinus says he was "keen and eager in plotting treachery", and describes him as "fickle flatterer" to Constantius II.
After the death of Constantius in 361ĕ he served Julian the Apostate and was chairman of the Chalcedon tribunal. In this function he was responsible for the conviction of Paul the Chain and many ministers and followers of Constantius.