Boris Ottokar Unbegaun (1898-1973) was a scholar of Slavonic languages, especially Russian. Together with William Kleesmann Matthews and Grigore Nandris, he is credited with bringing the philology of Russian and other Slavonic languages into academic prominence in Europe and in the United States.

Born in Russia, he attended schools in Moscow and St Petersburg, and studied in Ljubljana and at the Sorbonne. He held doctorates from the Sorbonne and the University of Oxford. In his earlier career he was a librarian with a significant bibliographical work published in 1929, Catalogue des périodiques slaves et relatifs aux études slaves des bibliothèques de Paris. Between 1936 and 1953, he taught at the University of Brussels and from 1946 to 1953 held a professorial position at the University of Strasbourg. He was the first Chair of Comparative Slavonic Philology at the University of Oxford (and Fellow of Brasenose College) from 1953 to 1965, and Professor of Russian Literature and Language at University College London from 1958. In 1965, Unbegaun became Professor of Slavic Linguistics at the Graduate School of Arts and Science of New York University.

He published numerous articles and monographs, including A Bibliographical Guide to the Russian Language (1953), Russian Versification (1956), Russian Grammar (1957) and Selected Papers on Russian and Slavonic Philology (1969). He was the general editor of the Oxford Russian-English Dictionary (1972).

He died at the New York University Hospital in 1973.