The article is the fi rst to publish a 15th century inscription from the Cathedral of St. Sofi a in Constantinople (Istanbul), made by a Russian clerk Ostafi y. The analysis of graffi to in the context of the Old Russian epigraphy of St. Sophia allows us to attribute it to the number of autographs of persons who visited the capital of Byzantium not for pilgrimage purposes, but on business. In connection to this the article attempts to compare the words dyakon and dyak, the latter of which became a name for a civilian clerk. The author’s professional status is revealed by the petition prayer, where he asks to strengthen his hand. It correlates with the records of 15th century Russian book scribes, which together make up an ideal portrait of a medieval scribe.