A court in the town of Amfissa convicted Epaminondas Korkoneas, 38, of intentionally killing 15-year-old Alexandros Grigoropoulos.
He was shot dead on 6 December 2008 in the Athens neighbourhood of Exarchia.
Korkoneas's patrol partner, Vassilios Saraliotis, 32, was given a 10-year jail sentence for complicity.
The riots that followed the killing saw cars being set alight and shops looted in a number of cities. Hundreds of businesses in Athens were targeted and the second city of Thessaloniki also saw serious unrest.
Further rioting took place on the first anniversary.
The verdict from a panel of judges and jurors was 4-3 in favour of convicting Korkoneas of intentionally shooting Alexandros.
Two judges and one juror had backed a lesser verdict of manslaughter with possible intent.
The nine-month trial heard that Korkoneas had fired three shots, during an altercation with youths on the streets of Athens.
His lawyer said these had been warning shots - responding to a hail of missiles - and cited an autopsy report indicating the boy had been hit by a ricocheting bullet.
However, witnesses and relatives testified that Korkoneas had deliberately taken aim and fired.
At the trial in January, Alexandros's mother, Gina Tsakilian, said the two defendants were "monsters in the guise of men".
Responding to the verdict, her spokesman, Capt Andreas Constantinou, said: "The family is happy with the outcome of the court proceedings. Justice has been done.
"Of course, Alexandros is not coming back, but at least what is important for the family is that his good name has been restored."
The trial was moved from Athens to Amfissa - a small town 200km (120 miles) west of the capital - to deter attacks by anarchist groups that had vowed to kill the two defendants.
Exarchia is a rebellious district, popular with self-styled anarchists, and there are frequent clashes with police.
The BBC's Malcolm Brabant in Athens says the chairman of the residents' association there, Manos Koufouglou, had told him he welcomed the verdict.
But Mr Koufouglou said that while tensions had eased, the people of Exarchia remained unhappy that the armed Special Guard unit to which Korkoneas belonged had not been disbanded.
"Police violence goes on," he told our correspondent. "The government has not done enough to reform the police.
"There will be a demonstration to mark the anniversary of the murder. We will not forget."