Eduard Palchys, 31, was found guilty of organising mass unrest and inciting hatred.
He joins a long list of designated political prisoners in the ex-Soviet country, with local rights group Vyasna saying 929 are currently behind bars.
Palchys was arrested in September 2020, shortly after nationwide protests erupted in the wake of a disputed election which Lukashenko claimed he won by a landslide.
Belarus's exiled opposition leader Svetlana Tikhanovskaya condemned the ruling and posted a video of Palchys in the defendant's cage in court.
"It was a closed trial, so nobody could see how the regime represses an innocent person," she said.
"Eduard's daughter was born when he was already behind bars," she said.
She added that "we won't stop until all political prisoners are freed."
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