Osip Aaronovitch Piatnitsky (Russian: Осип Аронович Пятницкий; Lithuanian: Josifas Piatnickis; born Iosif Oriolovich Tarshis;[1][2] 29 January [O.S.

17 January] 1882,[2] the son of a Jewish carpenter in the town of Ukmergė (then known as Vilkomir), in the Kovno Governorate of the Russian Empire (present-day Lithuania).

[3] Tarshis became a convert to Marxism and joined the Russian Social Democratic Labor Party (RSDLP) in 1899, moving that same year to Lithuania's largest city, Vilna (now Vilnius).

In 1901, Piatnitsky became associated with the Internationalist wing of the RSDLP, a group prominently including Vladimir Ulyanov (Lenin), which was at the time publishing the revolutionary newspaper Iskra from emigration in Germany.

[3] Piatnitsky became involved in the smuggling of this newspaper across the German frontier into Russia,[5] also helping to organize the transportation of party members to and from the country.

[3] He returned to Russia in 1913, taking a job as an electrician in the town of Volsk in Saratov Oblast, located on the banks of the Volga River.

[4] Piatnitsky was joined as a member of the Comintern Secretariat by the Bulgarian Vasil Kolarov, the Finn Otto Kuusinen, and Mátyás Rákosi of Hungary.

[4] Following the fall of Grigory Zinoviev in 1926, the post he formerly held as "President" of the Comintern was eliminated, to be replaced by a new Political Secretariat, to which Piatnitsky was elected.

[4] In addition to his leading role in the Comintern, Piatnitsky held several positions of high importance in the hierarchy of the Russian Communist Party.

[4] In the midst of the secret police terror known as the Great Purge, Osip Piatnitsky objected to the massacres, expressed doubt that charges against party comrades were valid when the Central Committee met in plenary session and was asked to sanction what had been happening.

As a result, in October 1937, like what happened to his comrade Kaminsky and others a few months earlier, he was removed from his position on the Central Committee, stripped of party membership, and arrested by the NKVD, the Soviet secret police.