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.
Principality Stadium will host the UEFA Champions League Final in six weeks' time. WRU chairman Gareth Davies, Football Association of Wales chief executive Jonathan Ford and Ambassador Ian Rush joined young sports fans at the trophy arrival at the venue to celebrate the magnitude of the event on Saturday 3rd June (kick-off 19.45).
British and Irish Lions captain Sam Warburton is looking forward to bringing together players from Ireland, Scotland, England and Wales for the biggest challenge of their careers - a series against world champions, New Zealand.
For the fifth year running, Wales referee Nigel Owens will be in charge of proceedings at Judgement Day V at Principality Stadium. It promises to be 'extra special' for the Wales' favourite ref, as he will be making his 150th appearance in the league.
There will be no shortage of inside knowledge from Bowl Finalists Caerphilly and Amman United when they face each other on Sunday at National Finals Day. Caerphilly hooker Rhys Cleverly knows the Principality Stadium pitch better than most as a matchday groundsman, while Amman United can look to Wales' record try scorer Shane Williams for some helpful advice.
Two hundred and 40 teams competed in the inaugural Urdd WRU 7s held at Pencoed last week with Welsh internationals Steff Andrews and Jasmine Joyce impressed with the talent on display, as was Urdd chief executive, Sioned Hughes.
The countdown is on for Judgement Day V as tickets for the big day at Principality Stadium soar through the 50,000 mark. Players and coaches alike are bursting at the seams to play in the double header with Dragons coach Kingsley Jones claiming there is a 'real buzz' about the event.
WRU TV went behind the scenes when the four regions got together to test their skill set for the BBC's Scrum V Live rugby show, at the WRU National Centre of Excellene in the build up to Judgement Day V. Blues' Matthew Morgan was one of the stars of the show while the Dragons' Cory Hill proved a surprise package...the full set of results will be revealed on tv screens on matchday
Wales assistant coach Richie Pugh is confident his side can cause a few headaches in the Hong Kong Sevens this weekend. In a tough group, Wales kick off their campaign against New Zealand on Friday before playing Japan and Fiji, who are chasing their third consecutive Hong Kong title, on Saturday.
Bedwas had to overcome a stubborn Carmarthen Quins side before winning the Foster's Challenge Cup final in Bridgend with a 23-18 victory. Bedwas captain Nicky Griffiths pays tribute to his side who have claimed the club's first piece of silverware in the modern era.
Team coach Brett Davey is aiming to keep his unbeaten run in cup finals at Principality Stadium intact when his side take on Amman United in the WRU Bowl final, after he guided Caerphilly to a 20-14 win against Cilfynydd in the semi-finals.
Penallta are hoping to go one step further than last season when falling to Bedlinog in the WRU Plate Final. After a titanic struggle against Nant Conwy, they have given themselves a great chance, after a hard fought 18-13 semi-final victory.