Legio I Flavia Constantia: one of the legions of the later Roman empire. Its name means "the reliable Flavian legion".
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 the east. It is nothing but a name to us. The name and its location, however, are suggestive, because the legion must have been founded when a Flavian emperor named Constantius ruled the eastern half of the empire. This must mean that it was founded by Constantius II (337-361).
This legion is not to be confused with I Flavia Gallicana Constantia, which served in Armorica in the west.