Bloody Wednesday (Polish: Krwawa środa) refers to the events of 15 August 1906 in the (Congress) Kingdom of Poland, where the Combat Organization of the Polish Socialist Party (OB PPS) carried out a series of attacks on Russians, primarily police officers and informants.

This took place in the context of the Revolution in the Kingdom of Poland (1905–1907), and represented one of the biggest actions in the history of OB PPS.

The aim of OB PPS was to stop repressions following in the wake of the revolution, and demonstrate its strength to the Russian government.

[13] Another large assault took place in Łódź, where a 25-strong OB PPS group under F. Lipiński attacked a police station, and ambushed several patrols.

[16][17] The Imperial Russian Army terrorized worker districts in Warsaw, Łódź and other towns;[18] the most infamous of the reprisal actions was the Russian-organized Siedlce pogrom on 8–10 September 1906.