The article describes the graphic and orthographic features of the unique Old Russian Psalter stored in the Vatican Apostolic Library (under the code Vat. slav. 8). This codex has practically not attracted the attention of researchers because it was difficult to access. It is now available on the Internet and can be fully studied. The Psalter was written by a highly qualified scribe who developed a special font for his manuscript — a half-letter, built following certain principles as the study showed. The orthography of the manuscript is focused on the ancient Russian norms of the 16 ^th century. However, it has very important innovations: superscripts appear that imitate a slight Greek aspiration, as well as the writing of omega and “uk” in the South Slavic manner. The localization and dating of the manuscript have been clarified. It was written no earlier than the 1420s as evidenced by the special “bead” initials, as well as the spelling of omega with a high center. Some features of the Central Russian dialect can be traced in the scribe’s writing. The codex was brought to Italy by Metropolitan Isidore who fled from Moscow (from the Chudov Monastery) in September 1441 with many other Greek and Slavonic manuscripts. As a result, this Manuscript can be associated with the scribes of this monastery. A small format of the Vat. slav. 8 suggests that this was a travel book. Apparently, the scribe created this manuscript for personal needs. He included a number of rare church services in the codex, in particular, the unique Tuesday church service of the akathist of the Blessed Virgin Mary according to the charter of Mount Athos.