Coach Chris Jones paid tribute to the "Rhondda spirit" shown by the squad as they made the most of their forward dominance to overpower a strong running Bridgend.
But it all could have slipped from their grasp had Bridgend not knocked the ball on with the line at their mercy in injury time.
It was an inauspicious start for the Rhondda boys as their opponents took advantage of the strong breeze at Llandovery to take a 10-0 lead.
Tom Prydie opened the scoring with a penalty then, 17 minutes in, Bridgend seized on another Rhondda mistake to score the final's first try,
When possession was lost on half-way, Tom Habberfield cut through to feed flanker Sam Treharne, who crossed under the posts. Prydie converted.
And Bridgend should have put Rhondda out of sight just before half time.
Prydie broke through and off loaded to Johnny Phillips, who knocked on five metres out.
With the last play of the half, Rhondda capitalised and used their forward prowess with a 20 metre rolling maul.
The defence eventually succumbed as Jack Gibbon was helped over the line.
Josh Wills converted to leave the valleys side 10-7 down at the interval.
If Bridgend had used the wind in the first period Rhondda did likewise in the second.
Outside half Lewis Williams kept Bridgend on the back foot with precision kicks to the corner and, following several rumbles by the pack, Rhondda scored again with a tried and tested forward effort.
Second row Tarian Emmanuel emerged with the ball to put his team 12-10 ahead, with Wills missing the conversion.
But just as time looked to have beaten Bridgend they had one last chance to win.
A break by Matthew Morgan was taken on by Michael Powell - but the winger's pass was dropped out wide with the try-line beckoning.
Rapturous celebrations from the Rhondda team and support ensued at the final whistle with Bridgend left to ponder what might have been.
An ecstatic Chris Jones said: "Of all the finals we've been in, that was the toughest. I don't think we played particularly well in the first half but I think the wind was a big factor. The boys showed a fantastic spirit to come back and win the game. To do that in a final from 10 points down shows they've got a lot of guts and commitment. I think they showed true Rhondda spirit at the end to come away with the victory. They've been a great squad of boys to coach. It has been a pleasure to work with them - and they deserve the shield. It was a squad effort to get us this far and the likes of Jake Thomas, Mitchell Auger, Lloyd Williams and Luke Matthews deserve special mention too."
Rhondda Schools under 15s 12 - Bridgend Schools under 15s 10
Jack Gibbon (1)
Tarian Emmanuel (1)
Josh Wills (1)