Mikhail Khodorkovsky, 43, who is serving an eight-year term in an East Siberian prison for fraud and tax evasion, was placed in a solitary confinement cell for 10 days in June 2006 for violating prison regulations against sharing food with fellow prisoners.
The appeal was considered without the presence of Khodorkovsky, who is being questioned in the regional center of Chita as part of a new criminal probe.
Since the oligarch's arrival at the prison in October 2005, he has been placed in solitary confinement four times, including on one occasion for his own safety after he had his nose slashed by a fellow inmate.
A court found that the first decision to place him in solitary confinement for allegedly possessing inappropriate documents was inappropriate.
"So far, two of the three [punitive] decisions on solitary confinement have been classified by the court as unjustified," said Natalya Terekhova, who represents Khodorkovsky.
Inmates who repeatedly violate prison rules can be denied the chance of early release.
The former tycoon, who acquired his company during controversial privatization deals in the early 1990s, has insisted his criminal case was orchestrated by Russian authorities to silence his criticism of President Vladimir Putin and as part of a campaign to bring mineral assets under Kremlin control.
The case is also seen by some experts as the Kremlin's backlash against Khodorkovsky's political ambitions.
