Flight FR4978 was en route from Athens to Vilnius when it turned east to Minsk shortly before it reached the Lithuanian border. Greece and Lithuania put the number of passengers on board at 171.
In a statement, Ryanair said that the crew had been "notified by Belarus (Air Traffic Control) of a potential security threat on board and were instructed to divert to the nearest airport, Minsk".
The flight path, visible on the Flightradar24 website, suggests the plane was actually nearer to Vilnius than Minsk when it turned.
Ryanair said checks in Minsk found "nothing untoward" and the aircraft left Minsk at 20:50 local time.
"We apologise sincerely to all affected passengers for this regrettable delay which was outside Ryanair's control," it added.