Marcus Calventius Viator was a soldier and commander of Roman Emperor Hadrian's horse guards, the equites singulares Augusti, during the early 2nd century. Viator served as a centurion in Legio IV Flavia Felix and was training officer (exercitator) of Gaius Avidius Nigrinus' horse guards (equites singulares). He made a dedication at the legionary base in northern Dacia at Apulum sometime between 110 and 118.^[1] In 118 Nigrinus was involved in a conspiracy with Lusius Quietus against Hadrian. It has been speculated that Viator may have implicated Nigrinus, given his later promotion under Hadrian.^[2] Viator was promoted to the emperor's horse guards sometime before 128, being attested at Zarai in Africa Province.