Merthyr survived a late onslaught to beat Rhiwbina 29-26 and take home the Swalec Plate at the Millennium Stadium.
A brace of tries from Owen Morgan and further scores from Josh Flye, Osian Davies and Gareth Way sealed a league and cup double for the Ironmen.
Rhiwbina got off to a flying start with tries Brett Chatwin and Sion Summers, but were out-muscled by the Division 1 East champions.
Louis Millar and Chatwin scored late tries to pull the Squirrels back to within three, but Merthyr survived a late scare to take home the Plate.
Rhiwbina stormed into the lead after a well-worked try from wing Chatwin. Outside-half Josh Davies executed a pinpoint cross-kick which found Chatwin, and the Squirrels flyer was too quick for the covering Merthyr defence.
The underdogs stretched their lead after 17 minutes with another score out-wide. The Rhiwbina backs spread the ball to the right flank and Chris Fido offloaded back inside for onrushing full-back Summers to crash over - Gareth McCarthy added the extras.
The Ironmen replied just moments later through full-back Fyle.
The Merthyr forwards tried to shove there way over before releasing the backs who pushed the ball to Fyle.
The full-back dummied his way through the scrambling Rhiwbina defence and touched down to pull his side back to within seven points.
Flanker Davies then found a gap in the opposition defence to complete Merthyr's 12 point comeback.
The back-rower darted through a hole in Cardiff side's defence to score from 10 metres out, fly-half Steve Lang added the extra two points to draw his team level on half-time.
Merthyr, who are a division above their Plate rivals, took the lead after 47 minutes.
The pressure finally told when Morgan squeezed in at the corner following Luke Harwood's offload, taking the score to 17-12.
Morgan scored his second soon after, the lightning winger rounded his opposite man and dived over in the same corner.
Way then crashed over for Merthyr's fifth try to seal the win.
Millar and Chatwin scored late tries to take their team to within three points, but they couldn't get over the line for the winning score.
The Foster's Challenge Cup will involve the 12 Principality Premiership sides and will run during European and British & Irish Cup weeks, providing vital fixtures for the participating clubs and an opportunity for upcoming players to step up to semi-professional rugby and make a claim for starting places within the Premiership squads.
Eighty schools and colleges in Wales now have a full time rugby officer as part of the WRU's school club hub scheme. All of the school club hub officers recently gathered at the National Centre of Excellence for various workshops.
The WRU has launched a campaign to create a long term legacy for Welsh club rugby by highlighting the advantages of volunteering. Rhian Edwards, a volunteer at Seven Sisters, has enjoyed many benefits of her volunteering at a grassroots rugby club including being part of the Rugby World Cup volunteer workforce after being nominated by the WRU - and the WRU is asking for more people to develop their 'Welsh rugby roots'.
The top three teams of the Principality Premiership put their unbeaten runs on the line this weekend. WRU TV's Graeme Gillespies is joined by South Wales Argus rugby writer Chris Kirwan to discuss the weekend's action
Geraint John has taken up the new role of WRU elite coach development manager having arrived in Wales last week. WRU TV caught up with him at the WRU rugby department's strategy day at the Millennium Stadium.
The entire Welsh Rugby Union Rugby Department gathered at the Millennium Stadium earlier this week where WRU Head of Rugby Josh Lewsey looked back on a year where structural and strategic changes have seen significant progress made
Tomorrow will see the first six-point try scored in the northern hemisphere when the Principality Premiership kicks off. WRU TV's Graeme Gillespie catches up with WRU National match Officials Manager Nigel Whitehouse, and Pontypridd winger Alex Webber as they look ahead to the innovative changes.
The annual Women's club launch was held at the Wales Centre of Excellence where National Women's Head Coach and Programme Lead Rhys Edwards, Competition Secretary Adrian Howell and Cardiff Met player Ffion Jones express eagerness for the new campaign to get under way.
Plenty of award winners in this week's Principality Welsh Rugby Union Podcast.Dragons centre Tyler Morgan wins one of the top awards for up and coming talent with the SportingWales Rising Stars 2015 award. Scarlets and Wales outside half Hannah Jones is one of the runners-up. We also he from community clubs as the season approaches and Principality Premiership unsung hero Ron Taylor of Ebbw Vale.
Actor and Millennium Stadium Tour Guide Gareth Bale has been performing his one-man play about Wales rugby legend Ray Gravell across Wales. He chats to WRU TV before taking his show to the Edinburgh Festival.
After widespread consultation, the WRU has made major changes to its pathway programme for girls rugby with a spring and summer season being introduced with immediate effect in order to respond to a clear demand to play the game.
The Webb Ellis Cup visited Lampeter on Monday to recognise the fact that the town is celebrating a major milestone as it is believed the first ever game of rugby played in Wales, was in fact played in Lampeter 150 years ago.
We cover the big news the 2017 Champions League Final will be coming to the Millennium Stadium, in this week's Principality Welsh Rugby Union Podcast. We hear from FAW chief executive Jonathan Ford, First Minister Carwyn Jones and WRU group chief executive Roger Lewis. Wales coach Warren Gatland describes their high altitude training in Switzerland, national match officials manager Nigel Whitehouse talks about the community coaches and referees conference, while we have a special feature on Cardiff Blues U18 girls sevens squad preparing for their first Gemau Cymru, with some help from internationals Elinor Snowsill and Philippa Tuttiett.
Bridgend Ravens continued their remarkable form against Premiership champions Pontypridd with four wins in five matches throughout 2015 after securing a 14-8 victory at Sardis Road in a Foster's Challenge Cup clash.