Neath ... 32 Cardiff ... 10Outhalf Lee Jarvis bagged 17 points to book Neath a place in the Celtic League final against Munster at the Millennium Stadium on February 1st.
Jarvis scored one of Neath's four tries and kicked three conversions and two penalties as the power of the Welsh All Blacks proved decisive in an entertaining all-Welsh clash at The Gnoll.
Both sides had carved out impressive victories on their travels in the quarter-finals - Neath at Pontypridd and Cardiff at Edinburgh.
After a period of sustained pressure on the Cardiff line it was Jarvis who opened the scoring with a 15th minute penalty. Then, with the Cardiff scrum creaking, number eight Nathan Bonner-Evans drove over from close range for the game's opening try.
With Neath continuing to test the Cardiff defence it was Tongan centre Dave Tiueti who crossed after a clever midfield switch to take the home side into a convincing 17-0 interval lead.
Cardiff were offered some hope three minutes after the restart with a try by scrum-half Richard Smith.
Outhalf Iestyn Harris then added a penalty to his earlier conversion to make it 17-10 and give Cardiff renewed hope of a comeback, similar to the one that earned them the win in Edinburgh.
But a moment of magic by Jarvis, dummying a dropped-goal attempt then racing over, put Neath back in control. The number 10 added another penalty and Gareth Morris a try to seal Neath's victory.
NEATH: Durston; G Morris, Storey, Tiueti, S Williams; Jarvis, A Moore; D Jones, B Williams, Millward, A Newman, G Llewellyn, Tandy, Bonner-Evans, Mocelutu.
CARDIFF: R Williams; Walne, J Robinson, Allen, Morgan; Harris, Smith; Payne, Woods, Fourie, Sennekal, Tait, Baugh, M Williams, E Lewis.
Referee: A Lewis (Ireland).