The Jewish Combat Organization (Polish: Żydowska Organizacja Bojowa, ŻOB; Yiddish: ייִדישע קאַמף אָרגאַניזאַציע Yidishe Kamf Organizatsie ; often translated to English as the Jewish Fighting Organization) was a World War II resistance movement in occupied Poland, which was instrumental in engineering the Warsaw Ghetto Uprising. ŻOB took part in a number of other resistance activities as well. Notably, a second Jewish resistance organization called the Jewish Military League (Polish: Żydowski Związek Wojskowy, ŻZW), formed primarily of former officers of the Polish Army in late 1939 – and operating side by side with ŻOB – was at least equally instrumental in the Jewish armed struggle.
The seeds of the ŻOB were planted on 22 July 1942, when the German Nazis under SS General Jurgen Stroop began the Gross Aktion Warschau sealing the fate of the Jews confined in the Warsaw Ghetto: "All Jewish persons living in Warsaw, regardless of age and gender, [would] be resettled in the East." Thus began massive "deportations" of about 254,000 Jews, all of whom were sent to the Treblinka extermination camp. The Gross Aktion lasted until 12 September 1942. Overall it reduced the once thriving Warsaw Jewish community of some 400,000 to a mere 55,000 to 60,000 inhabitants.