The legacy of St. Demetrius of Rostov (1651–1709), metropolitan of Rostov and Yaroslavl, an Orthodox saint and one of the most prominent Slavonic authors, has always attracted a great deal of attention. His works have been the object of study of both Russian and foreign scholars. This paper puts forward one of the most topical issues in the study of Demetrius’ writing, namely, his unresearched, unpublished, and previously unavailable works. These are discovered in manuscript collections which were compiled in and emerged from the Rostov scriptorium (some of them are original manuscripts from the metropolitan’s library). Among them are works that were unpublished in the first edition of the Lives of Saints by St. Demetrius of Rostov. The author of the present paper makes available a new, unknown, and unpublished work by St. Demetrius of Rostov, On the Year of Death of St. Mary of Egypt, which was written as an additional calendar text referring to the Life of St. Mary of Egypt. The sources of the Life of St. Mary of Egypt were the Acta Sanctorum and De Probatis Sanctorum Historiis by Laurentius Surius. The text On the Year of Death of St. Mary of Egypt was written by Demetrius for the first edition, but it was not included in the printed edition and is preserved only in manuscript sources.