The treaty, known as New START, limits each country to 1,550 strategic warheads, down from the current ceiling of 2,200, and also re-establishes a system for monitoring that ended in December 2009 with the expiration of a previous arms deal.
In a statement to his security council on Friday, Medvedev said the pact will take effect when the ratification documents are exchanged by Russian Foreign Minister Sergey Lavrov and American Secretary of State Hillary Clinton.
The pact was approved by the U.S. Senate last month after President Obama pressed strongly for its passage, telephoning a handful of hesitant Republicans to lock in their votes.
Democrats sought to appease some Republican senators by allowing them to raise concerns about the treaty in an accompanying resolution. The resolution didn't affect the text of the treaty, but Russian legislators felt compelled to offer their own interpretation of the pact's provisions in their ratification bill and accompanying statements.
While the Senate resolution said the treaty shouldn't restrict U.S. plans to develop a missile defense system, the Russian ratification bill states that the treaty can only be fulfilled if emerging missile defense systems don't erode the Russian nuclear deterrent.
The Russian bill also mimics the Senate resolution's concerns that the remaining nuclear arsenal is effective by emphasizing the need to modernize Russia's nuclear forces.
Defense Minister Anatoly Serdyukov said Wednesday that Russia was working to develop its own missile defense system, but didn't give any specifics.
Addressing concerns about Russia being forced to disarm under the treaty, he said that Russia now has a significantly smaller number of missiles and bombers than the treaty allows anyway.
Aging Soviet-built-missiles still form the core of Russia's nuclear forces, and the military has struggled to build their replacement.
NATO has approved a plan for a U.S.-led missile defense in Europe last fall and invited Russia to join. Medvedev was receptive of NATO's proposal but has not made a definite commitment.