Risca of Division Four East upset league rivals Tredegar Ironsides to reach the Millennium Stadium for the first time in their history.
The Gwent club, who produced Wales captain Ryan Jones, beat the promotion-bound Ironsides 13-12 having held the lead for most of the Swalec Plate semi-final.
Chris Fraser scored Risca's only try of the match on 20 minutes, it converted by Rhys Carpenter who also kicked two penalties.
Ironsides replied with two tries from Jonathan Mills and Mark Williams, the second converted by Ryan Hill, but although this pushed them back to a point deficit the Cuckoos refused to budge.
Risca backs coach Gareth Evans said: "Ironsides beat us twice in the league, but superb support helped us make it third time lucky.
"We are hoping that Ryan Jones will make an appearance to wish us well before the final, should his Ospreys and Lions commitments let him."
There they will meet Division Two West opponents Ammanford, who beat Two East club Rhydyfelin 26-12 in the other semi-final.
Tries from Shaun Miles, Matthew Gibson and Mike Evans saw them reach the final, despite Ross Lucas and Scott Perry crossing in reply for Rhydyfelin.
SWALEC Bowl Round-Up
Lampeter Town, one of the founder members of the WRU, reached the home of Welsh rugby by making it through to the SWALEC Bowl final.
Lampeter, who claim to be Wales's oldest club, booked their place at the Millennium Stadium showpiece in style, beating Glyncorrwg 48-3.
Mark Saunders ran in the first of their five tries before Llyr Jones and Joe Thomas both crossed during a lengthy stoppage time period at the end of the first half.
Aled Thomas and Gareth Griffiths added further tries early in the second half to guarantee the Division Three West club their first trip to the national stadium.
In the other semi-final, Division Three South West side Morriston saw off Pontyclun of Division Five South East in a thrilling 25-18 victory.
The two sides were neck-and-neck for most of the match, which was locked at 18-18 and heading towards extra-time.
But Sean Richards crashed over for his second try of the match in the final minute, converted by Ceri Palmer, to send his side into their first Millennium Stadium final.
Morriston manager Vince Matthews said: "It was an absolutely tremendous game, a fabulous spectacle."