The decision to uphold the 18-year, 10-month sentence of imprisonment was made Thursday.
Father Santoro was killed in February 2006 as he was praying in his church in Trabzon.
Last year, a Trabzon court found Oguzhan Akdin guilty of premeditated murder, illegal possession of a firearm and endangering public security.
Akdin, who was 16 at the time, was arrested shortly after the murder, still in possession of the weapon used in the crime, authorities said.
Akdin claimed his action was in response to the publication of cartoons about Mohammed by the Danish newspaper Yyllands-Posten, which were reprinted by other European publications.
ZE07100504 - 2007-10-05