The study of the Jewish presence in early Eastern Europe has been severely hindered by the ambiguity of medieval nomenclature for Old Rus'. Some scholars even went so far as to deny any connection between the medieval Hebrew term Rusi(')a(h) (in its graphic variations) and the principalities of Rus'. This essay examines this term and others denoting Old Rus' in the medieval Jewish tradition and traces their historical development: from rendering the authentic East Slavic self-appellation (Rus and Rusi) and Bible-based ethnology, either Byzantine (Ros/Rosh) or original (Tiras/Tiros), featured in texts of the tenth century; to ethnic (Canaan, i.e., "Slavia," 11th c.), ethno-confessional (Canaan of Yavan "Greek Slavia," 11th c.), and purely confessional definitions (Yavan "Greece," i.e., "the land of the Greek rite," 12th c.); and, finally, to the inflected term—Rusi(')a(h) or Rosi(')a(h)—which becomes universally dominant in the later Hebrew texts regardless of their provenance. The essay not only helps to resolve a cardinal problem of attribution for the earliest sources attesting to a Jewish presence in Eastern Europe but also reconstructs the history of an important Hebrew ethno-toponym. The proffered solution takes account of the historical contexts, exegetic traditions, and political and confessional perceptions of Old Rus' on the part of medieval Jews from diverse communities spread across Europe and the Middle East.