This paper examines the formative intellectual trajectory of the Greek Orthodox scholar Georgios Koressios (c. 1570-1659/60) and situates his education in Renaissance Italy within the broader framework of the early modern Republic of Letters. Focusing on the years he spent in Italian centers of learning, I will investigate how the intellectual milieu of late Renaissance Italy shaped the development of Greek Orthodox scholarship in the Ottoman Empire. While the migration of Byzantine scholars to the Italian peninsula after the fall of Constantinople and their contribution to the Renaissance have been extensively studied, the reverse dynamic—the impact of the Italian scholarly environment on Orthodox intellectuals who later operated in Ottoman context(s)—remains comparatively underexplored. Drawing on approaches from connected history and the history of knowledge, I will analyze Koressios as a mediator between intellectual worlds. His studies in Italy placed him in contact with humanist scholarship, Aristotelian debates, and emerging philological practices that characterized late Renaissance intellectual culture. These experiences profoundly influenced his later activity as "megas theologos" of the Patriarchate of Constantinople and as the author of a substantial corpus of theological and polemical writings circulating primarily in manuscript form. By taking Koressios as a study case, this paper contributes to a more decentralized understanding of the Republic of Letters, demonstrating how Orthodox intellectuals from the eastern Mediterranean participated in—and reshaped—the transregional scholarly exchanges that characterized early modern European intellectual life. Koressios thus emerges not only as a leading Orthodox theologian but also as an important agent in the transmission and adaptation of Renaissance intellectual traditions across confessional and imperial boundaries.