Alexei Navalny been given nine years in a “strict regime penal colony” in a fraud case rejected by supporters as fabricated.
Navalny was detained when he returned to Russia last year, after surviving a poisoning he blamed on the Kremlin.
He is already serving three and a half years in jail for breaking bail conditions while in hospital.
A judge has now found him guilty of fraud and contempt of court.
Prosecutors accused him of stealing $4.7m (£3.5m) of donations given to his now banned organisations, including his anti-corruption foundation.
Delivering her verdict, Judge Margarita Kotova said Navalny had carried out “the theft of property by an organised group”.
The new sentence replaces his earlier jail term, so the opposition leader will now have to serve some seven years in a maximum-security prison, with much stricter conditions and far more remote than the jail in Pokrov east of Moscow where he has spent more than a year.
A visibly gaunt Navalny folded his arms and exchanged comments with his lawyer as the ruling was read out.
Accusing the authorities of jamming his “last word” in court, he tweeted that he and his supporters would continue to fight censorship to “bring the truth to the people of Russia”.
8/10 On March 5, I presented to you the “Popular Politics” channel – and now it already has almost a million subscribers. Join us – together we will make the best media in Russia: https://t.co/A325d9useZ
— Alexey Navalny (@navalny) March 22, 2022