Osip Aaronovitch Piatnitsky (Russian: Осип Аронович Пятницкий, Lithuanian: Josifas Piatnickis; Iosif Aronovich Tarshis, 29 January 1882, Kovno Governorate – 29 July, 1938, Moscow), was a Russian revolutionary and Soviet politician. 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.