Two tries by Cardiff fullback Jamie Roberts and one each for Lee Williams and Jonathan Edwards were enough to ensure that coaches Dai Rees and Gareth Baber will return with their team with the Plate title from Hong Kong and the Bowl from Adelaide.
The Tongans had beaten England in the Pool stages and lost by only a point to eventual Cup winners Fiji. That made them favourites to beat Wales in the Bowl final, but they were unable to match the Welsh flair and resilience despite two tries from Vungakoto Lilo.
"We were disappointed not to qualify for the Cup competition, but to turn things around on Day 2 again and come up with another trophy is fantastic. The boys were out on their feet at the end, but they deserved their success," said Rees.
"It was an eighth final for us in as many tournaments since Wales returned to the Series, but we have ambitions beyond the Plate and Bowl. If we could keep a core squad of players together for more than one or two tournaments then I think we could break into the top eight - if not this season then certainly next year."
Wales beat Portugal 24-7 and then France 19-14 to reach the Bowl final. The Tongans beat Japan and Argentina, the team Wales had beaten in the Plate final in Hong Kong, to make it through.
Wales Sevens Squad in Adelaide: