A victory for the Ospreys will see them into the March 5 final, at Pontypridd, whilst a losing bonus point means try count would determine whether the Ospreys or Scarlets would go through to meet the North once more.
Tries from flanker David Barry, centre Sam Campbell and full back Josh Wills, plus two conversions and a penalty from outside half Jospeh Griffin put the hosts into a commanding 22-6 half time lead.
However, tries from centres Dale Ford and Luke Williams put the Scarlets within six points as the game moved into its final quarter.
Griffin kicked a penalty to make it 25-16 and that, in the end was enough, despite two further penalties from the Scarlets in the closing stages.
In the night's other game, there was more success for the Blues region, as although the Dragons led Blues South 8-7 at half time thanks to a try after 24 minutes from Matthew Jolly and a penalty from Lewis Roblin, it was the South that finished with the smiles.
Fred White scored a converted try for the South in the first period and after the interval, they ran away with maximum points courtesy of tries from wing Aled Williams, prop Joe Tomlinson and full back Dan Jones.
Gareth Evans struck back with a try for the Dragons, in the 70th minutes, but by then, South had done enough to secure another excellent victory.
Regional Performance Manager Gafyn Cooper said, "We're delighted that for the second year running the Blues North are into the final. Hopefully we can go one better this season and lift the title."
Once again the North have been the dominant team in the tournament throughout the group stage and have only lost one game in the last 15 WRU Championship fixtures. The South ended their season positively as well and finished on 17 points.
"The second half of the season showed real improvement and something to build upon for next year. The season's highs included two victories against the Dragons, a good draw away at the Ospreys and the privilege of being the only team to have beaten the North this season" said Cooper.