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 Half a Game initiative is currently sweeping across Wales and proving a major success. The scheme aims to give every young player at least half a game every weekend. WRU TV recently visited a festival in Narberth where clubs have fully embraced the initiative.
Both Bargoed and Swansea were seeking their first win of the Principality Premiership season this afternoon. It proved a fruitful day for the hosts - not only did they win, but Bargoed claimed a try bonus point.
Cardiff managed to keep their unbeaten record at home intact, but only just as Bridgend made them work extremely hard before the Blue and Blacks secured a hard fought Principality Premiership win at the BT Sport Arms Park last night.
Former Wales back row Andy Powell gives a guided tour of The Wern, home of Merthyr RFC, who are determined to prove a lot of people wrong and make their mark in their first season in the Principality Premiership.
Wales Sevens head coach Gareth Williams looks back on the Rio Sevens success where he helped GB win silver. He believes the successful campaign is a huge boost to his Welsh squad as they prepare for another World Series, as WRU TV's Graeme Gillespie discovers
British and Irish Lions head coach Warren Gatland will be heading back home to take on the All Blacks next year where for him, success for 2017 is all about going to New Zealand and winning a Test Series.
Last week all roads led to Eugene Cross Park where defending champions Ebbw Vale hosted this season's launch of the Principality Premiership. WRU TV's Graeme Gillespie was there to capture the thoughts of Damien Hudd, captain of Ebbw Vale, and WRU Head of Performance, Geraint John.
Llandovery made it three in a row when they defeated Aberavon in the final of the National Foster's 7s at BT Sport Cardiff Arms Park on Sunday. On another eventful day of sevens, Newport claimed the Plate and the Dragons won the women's crown
Last week saw Cardiff welcome players from around the world for the 21st Golden Oldies Festival. The tournament features players over 35 years of age, and this year's event saw a select few represent the WRU in a 'friendly' against the Wooden Spoon.
Wales Under 18 captain Callum Bradbury is delighted to have seen his side claim a narrow 18-17 victory against a strong SA Schools 'A' side to end the three-match Under 19 International Series in South Africa on a high note.