The tournament, which also involves Fiji, doubles as a qualifier for the World Cup with the combined 2016 and 2017 results deciding which two teams from the region win automatic berths in Japan in 2019.
It was the first test Tonga have hosted in eight years and drew a crowd of around 10,000 at the newly-refurbished Teufaiva Stadium in the Tongan capital.
Tongan halfback Sonatane Takulua scored all of his side's points in the second half with a converted try and a penalty as they built on their 20-16 lead at the break to hold off a late charge from the Samoans.
