President Lukashenko allowed to travel to Europe after political prisoners freed.
Lukashenko said the creation of a joint air-defence field should be part of a package toward “deepening military-technical cooperation”.
Both nations have opposed Nato’s expansion into former Soviet regions, and plans by the United States to create missile defence sites in Poland and the Czech Republic.
General Alexander Zelin, the Russian airforce chief, was quoted by local news agencies as saying that the deal would help Russia and Belarusia to strengthen the monitoring of their airspace.
He said the system would include five air force units and 10 air defence missile units.
Moscow has been a key sponsor of Belarus in the past, supplying the nation with cheap energy and loans.
Belarus, which has been hit hard by the global financial crisis, secured a $2bn loan from Russia last year, and has requested another $3bn.
But Lukashenko has also made efforts to improve relations with the West, and was approved for a $2.46bn loan from the International Monetary Fund in January.