Last week, Russian prosecutors brought new charges against Khodorkovsky, 45, and his business partner Platon Lebedev, 41, of stealing 350 million metric tons of oil and laundering $28.3 billion.
The previous custody period was due to expire on August 2, 2008.
Khodorkovsky and Lebedev were found guilty of tax evasion and large-scale fraud by a Moscow court in May 2005 and sentenced to nine years in prison. The Moscow City Court later reduced their terms to eight years.
The two men are currently serving their sentences in East Siberia. Both men have maintained they are innocent of the charges, with Khodorkovsky claiming the sentence was retaliation for his support of Russia's tiny opposition movement.
The jailing of Khodorkovsky and other executives of what was once Russia's largest independent oil producer has been widely criticized in the West and seen as part of the Kremlin's drive to regain lucrative energy assets.