Legio I Flavia Martis: one of the legions of the later Roman empire. Its name means "the Flavian legion, dedicated to Mars".
This legion is mentioned in the late-antique text known as Notitia Dignitatum as one of the units belonging to the field army that was stationed in Gaul. The name strongly suggests that it was founded by the emperor Constantius I Chlorus (293-306), who belonged to the Flavian dynasty. Later members of the same house were Christians and would not have called a military unit after the pagan war god Mars.
The soldiers were probably called Martenses. Other units with these surname are known to have guarded the Rhine border and the Armorican coast. It is possible that these units were somehow related to the field army unit, but it is not known how.