Munir Thalib, who exposed abuses by the Indonesian military during the rule of former president Suharto, died of arsenic poisoning on a Garuda flight from Jakarta to the Netherlands in 2004.
The court's decision came after prosecutors filed a case review, presenting what they said was fresh evidence that Priyanto served Munir with a poison-laced drink while in transit in Singapore's Changi Airport.
The sentencing is being seen as a breakthrough for Indonesia's judicial system, which critics had accused of helping to cover up a conspiracy to murder Munir from within the country's security services.
|Pollycarpus had denied any involvement |
in Munir's death [Reuters]
During the case prosecutors presented evidence alleging that Priyanto was working on a special assignment from the Indonesian intelligence agency.
In December 2005 Priyanto, who was on the flight but not on duty, was found guilty of the murder.
But his conviction was quashed by the Supreme Court in October 2006.
Priyanto has always denied any involvement in Munir's death as have Indonesia's intelligence services.
Munir was Indonesia's most prominent human rights activist and was often targeted the country's powerful military and secret service.