Oskar Osipovich Gruzenberg (in German transliteration - Grusenberg; Russian: Оскар Осипович Грузенберг; 1866–1940) was a prominent Russian defense attorney.
Gruzenberg was born to a family of a Jewish merchant in Yekaterinoslav in 1866 . After graduation from a gymnasium (high school) in Kiev, Gruzenberg enrolled in Kiev University to study jurisprudence. He made a name for himself defending striking workers and members of the revolutionary parties. He also participated in the defense of Beilis in the infamous case of blood libel. He was a founder of Folksgrupe.