The Greek–Slavonic–Latin Lexicon compiled by the Ruthenian monk Epifanii Slavinetskii (d. 1675, Moscow) is a valuable historical witness to the development of written language in the Early Modern East Slavic world. This study represents the first in-depth exploration of the dictionary’s linguistic profile and structure. It addresses topics related to multilingualism, language attitudes, and language contact with reference to the dictionary’s lexical material, and presents a systematic analysis of its sources, which comprise Western European reference works as well as Greek and Slavonic manuscripts and printed texts. Emphasis is additionally placed on specifically Ruthenian elements on multiple levels of language. The investigation of these topics enables us not only to revise and substantiate certain assumptions about Epifanii Slavinetskii and his legacy, but also opens a new window onto the interaction of written idioms in the Early Modern Polish-Lithuanian Commonwealth and Muscovy.