Phosterius is venerated as a hermit of the Byzantine East who is commemorated on January 5. The surviving accounts of his life are sparse: tradition records that he withdrew to a lofty mountain, where he led a life of ascetic solitude, and that he became known for upholding the veneration of icons against the iconoclast heresy.
According to the tradition preserved in the synaxarion, Phosterius was sustained in his solitude by an angel, who provided for his bodily needs. Through the witness of his holy life and the wonders attributed to him, he is said to have drawn many people back to the Church from the heresy of Iconoclasm.
The historical details of his life are uncertain. Some sources place him among the hermits of Anatolia, in the wilderness of what is now Turkey, and assign him to the seventh century; the in-repo record classes him broadly within the Byzantine era without fixing a precise century. The scarcity and lateness of the evidence have led at least one scholar to doubt whether the account is historically reliable.