Goals from Finn Russell, Jamie Truckell and Harry Brown gave the Royals a 3-1 win over lower division Fleur De Lys Nelson at Westleigh Park.
Portchester had previously won the PYL U16 A Division title by beating Hurstwood 2-1 in a play-off at Front Lawn.
Both teams had finished the season level on points, with goal difference not coming into play in their age group.
Striker Blakeley Fairweather netted both goals in the play-off final - part of his 29-goal seasonal haul for the U16s.
A 30th of the season arrived when Fairweather made his debut for the Royals’ U18s against Stoneham in a Hampshire Development League game recently.