Russia's lower house of parliament on Friday, for the first time, officially blamed Soviet dictator Josef Stalin for the 1940 massacre of more than 20,000 Polish officers and other citizens at Katyn.
Warsaw welcomed the statement as "a good step," just weeks before Russian President Dmitry Medvedev's planned visit to Poland in early December.
Although post-Soviet Russia had acknowledged the killings were carried out by Stalin's secret police, Friday's vote formally reversed years of Soviet propaganda that had blamed the mass killings on the Nazis.
Russia's communist party opposed the declaration.
The declaration is expected to go a long way toward repairing relations with Poland, which have declined since President Lech Kaczynski and other top Polish leaders died in a plane crash in Russia earlier this year. The Polish party was heading at the time to the site of the Katyn massacre for a memorial ceremony.
The Nazis overran Katyn after invading the former Soviet Union in 1941.