When the Bolsheviks revolted, the yeshiva was forced to flee from Lithuania to Kletsk, Poland. During his three years in Kletsk, Feinstein attended the famed Talmudic lectures of Rabbi Meltzer and his son-in-law, Rabbi Aharon Kotler. Then he transferred to the Mir yeshiva, where he became a leading student of Yeruchom Lebovitz and learned together with Yechiel Michel Schlesinger, future rosh yeshiva of Yeshivas Kol Torah in Jerusalem, and Yonah Karpilow of Minsk, who was killed in the Holocaust and whose "Yonas Eilem" was published posthumously. At this time, Chaim Shmuelevitz and Aryeh Leib Malin also studied in the Mir yeshiva. Despite being surrounding by notable scholars of the Torah, Feinstein was nonetheless thought of as "the genius of the yeshiva".