Russian Defense Minister Sergei Shoigu announced the deployment of the S-300 batteries on April 23 during a visit to Minsk for talks with Belarusian President Alyaksandr Lukashenka.
Shoigu also said the two countries are now working on plans to build a Russian military air base in Belarus.
He expressed the hope that the first aircraft could be deployed at the base by 2015.
He added that Russia will set up an aviation command office and deploy an unspecified number of combat fighter planes in Belarus this year.
Shoigu's comments come the same day that the Russia-NATO Council is meeting in Brussels to discuss, among other topics, Moscow's objections to the alliance's missile-defense plans in Europe.