Bishop Luigi Padovese, Vicar Apostolic of Anatolia, had his throat cut by his driver and bodyguard Murat Altun in the garden of his summer house in a seaside town near Iskenderun, in the southern province of Hatay.
The 63-year-old bishop, an Italian national who had led the Roman Catholic Church in Turkey since 2004, died on the way to hospital.
A Turkish court convicted 28-year-old Altun for the murder but rejected suggestions that it politically motivated or an organised crime attack, the source told AFP.
Turkish authorities have said Altun, who had been working for the bishop for four-and-a-half years, was suffering from psychological disorders.
Prosecutors had demanded life imprisonment.
There have been a spate of attacks in recent years against the tiny Christian minority in Turkey.
Last week, Turkish police arrested 13 suspects accused of plotting an attack against members of a Protestant church and their pastor in the nortwestern city of Izmit.
In 2006, 61-year-old Italian Catholic priest Andrea Santoro was shot dead in the Black Sea port city of Trabzon.
Three Protestants -- a German and two Turks -- were murdered at a Christian publishing house in the eastern city of Malatya in April 2007.
The same year, a young man stabbed and injured Father Adriano Franchini, a 65-year-old Italian, after attending Sunday mass at a church in the western city of Izmir.