There were four tries in total for no 8 Rory Gallagher, centre Lloyd Ryland, wing Daniel Robinson and full back Jonathan Spratt, and two conversions by Aaron Hopkins.
WYRU secretary Roger Thomas said, "Portugal played well and were very entreprising in attack - it was only after an hour's play that we dominated the game. It was a good test for our players and that is what this tournament is all about - giving the boys experience at this level."
Italy beat Spain 31-10 in yesterday's other game, also at Tonmawr.
The four teams play again on Wednesday at Pontypridd RFC - Italy play Portugal at 5.30pm followed by Wales v Spain at 7.30pm.