Bloody Sunday or Red Sunday (Russian: Крова́вое воскресе́нье, tr. Krovávoe voskresénje, IPA: [krɐˈvavəɪ vəskrʲɪˈsʲenʲjɪ]) was the series of events on Sunday, 22 January [O.S. 9 January] 1905 in St Petersburg, Russia, when unarmed demonstrators, led by Father Georgy Gapon, were fired upon by soldiers of the Imperial Guard as they marched towards the Winter Palace to present a petition to Tsar Nicholas II of Russia.

Bloody Sunday caused grave consequences for the Tsarist autocracy governing Imperial Russia: the events in St. Petersburg provoked public outrage and a series of massive strikes that spread quickly to the industrial centres of the Russian Empire. The massacre on Bloody Sunday is considered to be the start of the active phase of the Revolution of 1905. In addition to beginning the 1905 Revolution, historians such as Deakin and Lionel Kochan in his book Russia in Revolution 1890–1918 view the events of Bloody Sunday to be one of the key events which led to the Russian Revolution of 1917.