Father Andrea Santoro, 61, was shot dead at the Sancta Maria church in the Black Sea city of Trabzon in February.
The shooting occurred at a time of widespread anger in the Islamic world over caricatures of the Prophet Muhammad in European newspapers.
Witnesses said the youth, who has only been identified in the Trabzon court by his initials OA, shouted "Allahu Akbar [God is Greatest] before firing two bullets into Santoro's back.
The court said the boy had been found guilty of "premeditated murder, possessing a weapon without a licence and endangering public security".
The boy's family criticised the sentence as being too heavy and his lawyer said he would appeal.
Santoro's murder led to public outrage in Muslim-majority, secular Turkey, which is seeking to join the European Union and has always prided itself for its religious tolerance.
The killing also caused an uproar in Italy, where the church announced during Father Santoro's funeral that it intended to have him proclaimed a saint and martyr.
After the killing of Santoro, two more Catholic clerics were attacked in Turkey.
A group of young men threatened a Catholic priest in the Aegean port city of Izmir.
The priest, a Slovenian, told Italian state televsion at the time that the men grabbed him by the throat, threw him into a garden and threatened to kill him.
The third attack occurred in July when a man stabbed a Roman Catholic priest in the Black Sea port of Samsun.
The French priest, Pierre Brunissen, 74, was injured in the hip and leg.