Coach Mana 'Otai named a strong Tonga team and they went 7-0 up midway through the first half when flyhalf Kurt Morath, who missed a penalty earlier, scored a try and booted the conversion.
Romania hit back with two penalties from flyhalf Florin Vlaicu, the country's all-time leading points scorer, and went to the break trailing 10-6.
The visitors grew even more confident in the second half and scrumhalf Sonatane Takulua touched down before hooker Andrei Radoi replied with a try for the home team.
Tonga have been drawn in World Cup Pool C alongside New Zealand, Argentina, Georgia and Namibia while Romania are in Pool D with France, Ireland, Italy and Canada.
The Romanian Rugby Federation is to make a donation from Saturday's ticket sales to former Tonga flanker Sione Vaiomounga who is suffering from kidney failure.
Vaiomounga played for Romanian champions CSM Baia Mare.