Flavianus was a Roman civil official who served as governor or prefect of Egypt (praefectus Aegypti) from 364 to 366. He was a native of Illyricum, the only one since the emperor Constantine I to hold the office. 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. Flavianus was succeeded as prefect after 21 July by Proclianus.

• Jones, A.H.M.; J.R. Martindale & J. Morris (1971). "Flavianus 3". The Prosopography of the Later Roman Empire Volume 1: A.D. 260–395. Cambridge University Press. p. 343. ISBN 0-521-07233-6.
• Lenski, Noel (2003). Failure of Empire: Valens and the Roman State in the Fourth Century A.D. Berkeley: University of California Press. p. 61. ISBN 0-520-23332-8.