Foreign Ministers of Russia and Belarus signed an agreement on mutual recognition of visas. With a new agreement, Russia will start letting foreigners with Belarusian visas in to the country, and Belarus with Russian visas.
[Russian Immigration at Moscow airport]
Travelers who have received a Belarusian visa will be able to enter the territory of Russia, and those who received a Russian visa will be able to enter Belarus. Visas will be mutually recognized for stays of up to 90 days.
Note that since Belarus allows citizens of certain countries to travel either visa-free or visa on arrival for 30 days, this rule does not apply to that category. Only travelers with either countries’ visa will be applicable to travel to the other country.