"A European missile defence system can become truly effective and sustainable only in the case of equal participation of Russia," the Kremlin press service said in a statement, citing a letter by Medvedev to members of the NATO-Russia Council.
In November, after years disagreement, NATO and Russia agreed to cooperate on building a common defence system against missiles fired by countries seen by the West as "rogue states," such as Iran.
But Russia and NATO remain in dispute over the implementation of the system. While NATO would like to see it and Russia develop two separate shields that exchange information, Russia argues for a single joint system.
President Barack Obama pleased Russia in 2009 by scrapping the previous US administration's plans to base interceptors for long-range missiles in Poland and a powerful radar in the Czech Republic. The United States has said it plans to deploy shorter-range interceptors in Eastern Europe.
A surprise decision by Washington to base anti-missile interceptors in Romania angered Russian officials, who said that Obama was pressing ahead with missile-shield plans despite promises of cooperation with Moscow.
Medvedev's statement on Saturday comes two weeks before Obama is due to visit Poland, where he is expected to confirm a deployment of American fighter jets and discuss basing interceptor missiles there.
The US expansion of its military presence to yet another former Soviet satellite state is likely to draw further criticism from Moscow.
On Monday, a group of Russia's experts, headed by Moscow's envoy to NATO Dmitry Rogozin, is to discuss the development of the European missile system.
Medvedev also said in the statement on Saturday that the deployed missile system cannot be directed against any of the parties and cannot undermine strategic stability.
Russia has long expressed the worry that US anti-missile defences could one day neutralise its own strike capabilities, triggering a Cold War-style arms race.
Medvedev and Obama are likely to talk about the future of the European missile defence system on the sidelines of the Group of Eight meeting of industrialised countries in France later this month.