"Mikhail Khodorkovsky was released from an isolation cell at 9.00 a.m. local time [midnight GMT]," his lawyer Natalia Terekhova said.
The jailed oligarch, serving an eight-year sentence for fraud and tax evasion in the Chita Region, was put in the isolation cell for "eating outside the designated premises."
Khodorkovsky's lawyers said they would appeal the decision, saying the prison authorities "did not have sufficient grounds" to put him in solitary confinement.
It was a second time that the former Yukos head has been placed in an isolation cell. He was first sent there for five days on January 24 for keeping documents not allowed to prisoners.
Khodorkovsky and his business partner Platon Lebedev were found guilty by a Moscow court in May 2005 and sentenced to nine years in prison. On September 22, the Moscow City Court reduced their terms to eight years.
Khodorkovsky is serving his sentence in a penal colony in the town of Krasnokamensk in east Siberia, while Lebedev is at a colony in the Arctic region of Yamalo-Nenets.