Osip Aaronovitch Piatnitsky (Russian: ???? ???????? ?????????; 1882 – 1938), born Iosif Aronovich Tarshis, was a Russian revolutionary.
Piatnitsky is best remembered as head of the International Department of the Communist International during the 1920s and early 1930s, a position which made him one of the leading public faces of the international Communist movement.