Poczobut was arrested in March 2021 on charges of "instigating hatred against religious and national grounds and rehabilitating Nazism."
His closed-door trial began on January 16.
A well-known journalist in Belarus and a long-time correspondent for Poland's Gazeta Wyborcza newspaper, Poczobut was also an activist for the Union of Poles in Belarus (ZPB), a Polish minority organisation that has been delegalised by the Belarusian authorities.
Poczobut is internationally recognised as a political prisoner. Poland and other countries have repeatedly appealed for his release.
The Polish foreign ministry on Wednesday condemned the verdict, calling it "an unjust ruling by the court of an authoritarian state."
Lukasz Jasina, the ministry's spokesperson, called Poczobut "a Polish and Belarusian patriot," and said Poland will continue supporting him.
"We're standing and will continue to stand by him," Jasina said.
Poczubot's sentencing comes as the Belarusian regime, according to opposition groups, tightens its control over the country through the use of mass arrests and purges. (PAP)