Flavianus was a Roman civil official who served as governor or prefect of Egypt (praefectus Aegypti) from 364 to 366.
Flavianus was apparently already serving in some administrative capacity in Egypt, perhaps as praeses, before succeeding Maximus as prefect in 364.
On 5 May 365 he received from the emperor Valens an edict on banished bishops, and on 8 June he sent the emperor a report seeking instructions on how to deal with the trinitarian bishop Athanasius.
On 5 October 365, Flavianus, alongside the dux Victorinus, tried to arrest Athanasius, who escaped.
On 1 February 366, he was instructed, through the notary Brasidas, to desist and allow Athanasius to return.