Tsikhanouskaya, 40, a former English teacher, fled to neighbouring Lithuania in 2020 after running against incumbent leader Alexander Lukashenko in a presidential election, which official results showed Lukashenko won by a landslide.
She and the opposition said at the time that the results had been doctored to hand victory to Lukashenko instead of herself. Lukashenko, who has ruled Belarus with an iron hand for nearly 30 years, denied the claim.
Mass protests against Lukashenko, a close ally of Russia's President Vladimir Putin, then erupted which his security forces suppressed, locking up his opponents or forcing them to flee.
The authorities put Tsikhanouskaya, the opposition's de-facto head, on trial in absentia in January, accusing her and other opposition figures of trying to seize power in an unconstitutional way.
Belta, the state news agency, said a court in Minsk had sentenced Tsikhanouskaya to 15 years in a prison camp after finding her guilty of treason and conspiracy to seize power.
The same court handed an 18-year prison sentence to Pavel Latushko, a prominent member of the Belarusian opposition council, and 12-year jail sentences to three other activists convicted of being part of the same plot, Belta reported.
"15 years of prison. This is how the regime 'rewarded' my work for democratic changes in Belarus," Tsikhanouskaya said on Twitter.
"But today I don't think about my own sentence. I think about thousands of innocents, detained & sentenced to real prison terms. I won't stop until each of them is released."
Rights activists estimate about 1,500 people are in jail in Belarus on politically motivated charges.
Tsikhanouskaya's own husband, Syarhei, is serving an 18-year jail term after being found guilty in 2021 of organising mass unrest in a ruling she said was political revenge and part of a crackdown by Lukashenko on anyone he viewed as a threat.
Nobel Peace Prize winner and human rights activist Ales Bialiatski was sentenced to 10 years in prison on Friday by a court in Minsk in a trial condemned in the West as a "sham".
Lukashenko, in power since 1994, has accused the West of trying to destabilise Belarus and promised to crack down hard on any new attempts to challenge his rule.
Reuters