Legio I Flavia Gallicana Constantia: one of the legions of the later Roman empire. Its name means "the reliable Flavian legion from Gaul".
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 suggests that it was founded by Constantius I Chlorus (r.293-306), who belonged to the Flavian family, and it has been assumed that the first assignment of this unit was to protect Armorican coast.
This legion is not to be confused with I Flavia Constantia, which served in the east.