After years of outright denial, Russia has finally admitted that Stalin directly ordered the mass execution of Polish officers during the Second World War.
Over 14,000 Polish military personnel died in the woods of Katyn, in western Russia in 1940 at the hands of the Red Army.
But until now, Russia has always blamed the Nazis for the atrocities.
The Duma is Russia’s lower house of parliament.
Russian President Medvedev released key documents earlier this year, establishing the guilt of Stalin.
The Duma has now formalised those documents in a bid to draw a line under the infamous case, which has been a bone of contention between Russia and Poland ever since.
For decades, the classified archive material was kept firmly out of the public arena.
A ceremony with Prime Minister Putin and his Polish counterpart Donald Tusk was held to mark the 70th anniversary of the massacre.
The move has been hailed as a positive step to improving relations between the two countries, ahead of a state visit by President Medvedev to Poland next month.