For many years, the Rare Books Department (Book Museum) of the Russian State Library has been conducting up-to-date work on the description of ex-libris, which contributes to the disclosure of the Department’s collections. The main goal of this research is to identify, record, study, publish, and thereby show the variety and richness of the ownership marks found on books. This article is devoted to the book marks of the German bibliophile Prince George III of Anhalt (1507—1553) from the collection of the Russian State Library. In the Russian-language research literature, Prince George’s book marks have not been considered before. The highly valued private library, later named after the owner — “Georgs-Bibliothek”, used to be part of the Land Library in Dessau (Germany). A small part of this famous book collection came after World War II to the V.I. Lenin State Library of the USSR and is now stored in the Book Museum. On the example of the small fragment of Prince George’s famous library, the article traces the gradual appearance and development of the unique ex-libris of this collection, reveals the literary and bibliophile interests of the owner, and establishes the circle of his communication. In the course of the study, about a hundred owner’s marks were recorded, thanks to which there were identified more than 120 publications from the personal collection of Prince George of Anhalt. The article presents the main types of its ex-libris (handwritten, gift, and super-ex-libris), which are reproduced and described in detail.