Mehmet Ali Agca, 48, served 19 years of a life sentence in Italy for attempting to assassinate the late pope in St. Peter's Square in Rome in 1981.
He was pardoned in 2000 and extradited to Turkey, where he has been serving a 10-year sentence at Kartal high security prison in Istanbul for murdering a Turkish journalist in 1979.
He is to be released early for good behavior.
Ferdinando Imposimato, the retired judge who investigated the murder attempt on the late pope, said Agca's life could be in danger because "he knows too much," The Times of London reported. He told The Times that he was convinced the murder had been planned in Moscow.
Investigators into the assassination attempt allege that Eastern European agents were involved, and that there were other gunmen in the square at the time of the shooting.
Three Turks and three Bulgarians were tried in 1986 for involvement in the plot, but were acquitted for lack of evidence.