Former tycoon Mikhail Khodorkovsky was released from jail post a pardon from Russian President Vladimir Putin, according tohis lawyers.
Mr Putin had signed a decree earlier to pardon Khodorkovsky on the basis of “the principles of humanity”.
On Thursday, the president said that the former oil magnate had asked him for mercy because his mother was ill.
Khodorkovsky remained in custody for ten years. He was jailed for tax evasion and theft after funding opposition parties.
The pardon follows amnesty backed by Russian MPs for at least 20,000 prisoners.
Analysts say Putin may be attempting to ease international criticism of Russia’s human rights record ahead of February’s Winter Olympics in Sochi.