Neath were thwarted in their bid for a sixth crown in seven years as Llanelli won 46-5 at the Gnoll in the Welsh Premiership play-off semi-final.
A hat-trick of tries from centre Nic Reynolds sealed a convincing win for Llanelli, who will now play top-of-the-table Pontypridd for the title.
Winger Chris Morgans' unconverted try was Neath's only response to the onslaught.
The final will be at Sardis Road on Thursday, 19 May.
Meanwhile, Cross Keys clinched Wales' final place in next season's British & Irish Cup by beating Newport 25-18 at Pandy Park in the qualification play-off final.