The Naro-Fominsk military court near the capital, Moscow, found Alexander Litvinenko guilty of beating up witnesses in his work as an officer of Russia's Federal Security Service (FSB), the KGB's main successor.
Mr Litvinenko - who was tried in absentia after refusing to return to Russia from the United Kingdom - denied all the charges through his lawyer and said he would appeal.
Mr Litvinenko first came to prominence in 1998 after telling the Russian media about an alleged plot by his superiors to assassinate Russian oligarch Boris Berezovsky.
Whistleblower
Mr Litvinenko was arrested in 1999 and spent nine months in jail on charges of abuse of office.
He later was acquitted of all charges and fled to London, where he was given political asylum.
Mr Litvinenko also wrote a book accusing his former bosses at the FSB of carrying out several apartment house bombings in 1999 that killed more than 300 people.
The attacks, which Moscow blamed on Chechen rebels, led to Russia's second war in the breakaway republic.
Mr Litvinenko's supporters called the charges against him an act of revenge for his whistleblowing.
They also said the trial of Mr Litvinenko and another former KGB spy had been carried out just in time before Russia's new Criminal Procedural Code took effect on 1 July.
The new code does not allow trials in absentia.
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