The Katyn massacre, also known as the Katyn Forest massacre (Polish: zbrodnia katyńska, literally 'Katyń crime'), was a mass execution of Polish citizens ordered by Soviet authorities in 1940. Estimates of the number of executed persons ranges from 15,000 to 21,768. Polish POWs and prisoners were murdered in Katyn forest, Kalinin (Tver) and Kharkiv prisons and elsewhere. About 8,000 of the victims were officers taken prisoner during the 1939 invasion of Poland, the rest being Polish citizens who had been arrested for allegedly being "intelligence agents, gendarmes, spies, saboteurs, landowners, factory owners and officials." Since Poland's conscription system required every unexempted university graduate to become a reserve officer, the Soviets were thus able to round up much of the Polish intelligentsia, as well as the Jewish, Ukrainian, Georgian and Belarusian intelligentsia of Polish citizenship.

The term "Katyn massacre" originally referred to the massacre, at Katyn Forest near villages of Katyn and Gnezdovo (about 12 miles (19 km) west of Smolensk, Russia), of Polish military officers confined at the Kozelsk prisoner-of-war camp. It is applied now also to the execution of prisoners of war held at Starobelsk and Ostashkov camps, and political prisoners in West Belarus and West Ukraine, shot on Stalin's orders at Katyn Forest, at the NKVD (Narodny Komissariat Vnutrennikh Del) Smolensk headquarters and at a slaughterhouse in the same city, as well as at prisons in Kalinin (Tver), Kharkiv, Moscow, and other Soviet cities.

The 1943 discovery of mass graves at Katyn Forest by Germany, after its armed forces had occupied the site in 1941, precipitated a rupture of diplomatic relations between the Soviet Union and the Polish government-in-exile in London. The Soviet Union continued to deny responsibility for the massacres until 1990, when it acknowledged that the NKVD secret police had in fact committed the massacres and the subsequent cover-up. The Russian government has admitted Soviet responsibility for the massacres, although it does not classify them as war crimes or as acts of genocide, as this would have necessitated the prosecution of surviving perpetrators, which is what the Polish government has requested. It also does not classify the dead as the victims of Stalinist repressions, in effect barring their formal posthumous rehabilitation.