Youth Front leader Zmitser Dashkevich was found guilty of participating in an unsanctioned rally in Minsk on April 26. The march was called to coincide with the 20th anniversary of the Chornobyl nuclear disaster.
Four opposition leaders, including former presidential candidate Alyaksandr Milinkevich, were convicted on similar charges on April 27.
The European Union and the United States have condemned the detentions.
The Belarusian opposition continues to protest the outcome of the March 19 presidential election that gave incumbent Alyaksandr Lukashenka a landslide victory. Western organizations and individual countries say the vote failed to meet international electoral standards.