The letter says Khodorkovsky is a political prisoner and that Putin is personally responsible for his survival.
The signatories include the head of the parliament's largest faction, the European People's Party.
The authors say the letter is timed to coincide with the G8 summit in St. Petersburg on July 15-17 and has been sent to all participating leaders.
MORE: To read more about this topic from RFE/RL's Russian Service in Russian, click here.