Russian National Guard troops can be deployed to Belarus to "secure public order" under a deal dated Friday and published by the Belarusian government.
There have been mass protests against Belarus' long-time president, Alexander Lukashenko, every weekend in Minsk since a disputed election five months ago.
Russia is neighbouring Belarus' closest ally. Russian President Vladimir Putin has pledged to send law enforcement officers to Belarus if needed to suppress the protests.
Lukashenko, 66, has led Belarus - a former Soviet republic in Eastern Europe bordering EU states Poland, Lithuania and Latvia - for more than a quarter of a century, tolerating little dissent.
The European Union has refused to recognize him as the current president, saying the 9 August election was "neither free nor fair."
Tens of thousands of protesters have been detained since the election, according to an assessment by the United Nations.