Russia's lower house of parliament has condemned Joseph Stalin by name for the mass execution of Poles at Katyn during World War II.
The Duma declared that the Soviet dictator and other Soviet officials had ordered the "Katyn crime" in 1940.
The statement, which comes weeks before a Russian presidential visit to Poland, was welcomed in Warsaw.
In a stormy debate, Communist MPs opposed the declaration, some seeking to deny Soviet guilt.
Soviet propaganda sought for decades to portray the massacre as the work of the Nazis, who overran Katyn after invading the USSR in 1941.
The truth was finally acknowledged in 1990, in the dying days of Soviet power, but the issue has continued to cloud relations between Russia and Poland.
The Duma said it hoped for "the beginning of a new stage in relations" with Poland "based on democratic values". Russian President Dmitry Medvedev is set to visit the country early next month.
Grzegorz Schetyna, Speaker of Poland's Sejm, the lower house of the Polish parliament, described the Duma declaration as a "good step and an important sign".
"President Medvedev's visit will thus take place in a better atmosphere," he was quoted as saying by AFP news agency.
"Published documents, kept in classified archives for many years, not only revealed the scale of this horrific tragedy, but also showed that the Katyn crime was carried out on direct orders of Stalin and other Soviet officials," the Duma declaration says.
"Official Soviet propaganda attributed responsibility for this villainy, which has received the collective name of the Katyn tragedy, to Nazi criminals.
"This theory remained the subject of hidden but nevertheless fierce discussions in Soviet society and unfailingly provoked the wrath, grievance and mistrust of the Polish people."
Russian leaders have publicly expressed regret for the massacre and this year saw the official online publication, by order of Mr Medvedev, of key documents proving the guilt of Stalin and his secret police chief Lavrenty Beria.
Nobody has ever been convicted over the massacre, with Russian prosecutors arguing that those responsible are now dead.
A Russian judicial investigation in 2005 only confirmed the execution of 1,803 victims, while the actual number of Polish prisoners killed at Katyn and other Soviet sites is generally held to be about 22,000, including about 8,000 military officers.
The Duma declaration called for the massacre to be investigated further in order to confirm the list of victims.
The Duma also argued that Katyn was a tragedy for Russia too as thousands of Soviet citizens were executed and buried in ditches there in the years 1936-38, the period of Soviet history known as the Terror.
Russia's Communist Party, which described Katyn last month as "one of the greatest myths of the 20th Century", voted against the declaration.
One of its MPs, Viktor Ilyukhin, told parliament the declaration was "degrading".
"It is alarming that for several decades, Russians have been forced to kneel and made to apologise for everything, even for things they did not commit, like apologising for the Katyn tragedy, which was not our fault," the Communist MP said.
But Konstantin Kosachev, head of the Duma's foreign affairs committee, said MPs had a duty to "remove this lie from our path".
"We want to close this issue, paying tribute to the victims of Katyn and condemning those who committed the evil deed," he said.
My great grandfather was killed in Katyn. He was one of the high rank Polish officers. His daughter, my grandmother, was left an orphan since her mother died during labour. She was sent to Siberia for several years and was dragged from one orphanage to another. Even though I welcome Russian acknowledgement of the Katyn massacre, after so many years of denial, my grandma cannot forgive them. My generation has forgotten the war as we have never known it. However, our grandparents have lived through it and only now do they get apology for their loss. For them it's simply too little too late. Karolina
Although no members of my family were killed in Katyn or other sites of this Soviet massacre (on the contrary, my grandfather was executed by the Gestapo), I think today's declaration of the Duma and the oncoming visit of president Dmitry Medvedev are crucial steps in the right direction. They will improve the relations between our two countries and help strengthen European integration. Everything takes time in this world, especially changing people's mindsets. In my student's years I happened to be in the Soviet Union a few times. Talking to Russian students I found them very nice, open and hospitable people, I could not believe that they really meant what they were saying about history and world politics. Revision of our political thinking is not an overnight matter. Michal Siuda, Poznan, Poland
My wife is Polish This means a great deal. Sometimes terrible things happen. It's helpful to acknowledge they were terrible and apologise, it really does help people move on. The British government recently apologised for Bloody Sunday (as indeed it should have done) and it helped. People aren't stupid and eventually the truth will come out. For me it signals that Russia is at last starting to be honest with itself and the world and that is a very good thing. Russia should be a friend - I hope they complete the journey. Ian, Leicester, UK
My father was not killed in Katyn, but suffered greatly because of it. He came to Canada and still lived a life of paranoia until his death in 1989. He was a part of the Polish resistance and aided in getting allied soldiers and pilots out of Poland. He was never feted for his accomplishments, as other allies were, and felt severely betrayed when Poland was "handed" over to the USSR. Mike Krawczynski, Toronto Canada
That is good news. I am really not sure that Russia is guilty. But it is our duty to be honest and have an open mind. The other step would be good if we gave some regret towards Polish people. No regret can put a shame on us. Ilya Laykin, Belgorod, Russia
I don't think the Duma decision is wise. In fact, if Stalin was not there to make Russia a superpower, the Duma would have been under the control of other countries. I am not defending what he did at Katyn, if he really did it. Georgian person
My uncle's father was killed in Katyn as he was a policeman in the Soviet-occupied eastern Poland. My uncle had to hide this fact in his personal files as he would not get a job in communist Poland, nor would he be accepted into university. I have known from my childhood who killed our best people, our elite - so although school history books told otherwise, we knew the truth. There was just no way we could believe the communists. Krzysztof Muchorowski, Warsaw, Poland
The truth prevails at last. That Russian communists should still refute the Katyn massacre is of no surprise to me. The Molotov-Ribbentrop Pact was a vicious agreement between two barbaric states to crush Poland as a country and to annihilate Poles. Both Soviets and Nazis did this with great zeal. Members of my family were murdered in both Auschwitz and Katyn. If the Russian Duma has after so many years decided to make truth prevail, it is only to their honour. Will this declaration help relations between Russia and Poland? I suppose it must; it should also reveal the truth of the numerous massacres to the Russian people themselves, who had to suffer so much be it under Lenin, Stalin, Khrushchev or Brezhnev. Ian Grocholski, Versailles, France