Ostap Stepanovich Parasyuk (born 20.12.1921) is a Soviet theoretical physicist and an academician of the Academy of Sciences of the Ukrainian SSR (1964). Born in the village of Belka (now in Lvov Region). Graduated from Lvov University (1947). In 1949–66 he worked at the Institute of Mathematics of the Academy of Sciences of the Ukrainian SSR. From 1966 he was head of a department of the Institute of Theoretical Physics of the Academy of Sciences of the Ukrainian SSR. At the same time in 1957–73 he was professor at Kiev University.
His main research was in quantum field theory and mathematical physics. Together with N. N. Bogoliubov he substantiated the computational procedure of quantum field theory (1955–60), proposed a new method of studying the analytic properties of the scattering amplitude (1962–65), solved a number of elastic-plastic problems (1947–49), and determined the spectrum of the horocycle flow on surfaces of negative curvature (1953).