Martyn Thomas crossed for a try to cut the deficit whilst Daniel Evans made sure of a tense second half with a conversion and penalty.
However, after the interval the Scarlets were outplayed by an Ospreys side who not only won but who achieved victory with a bonus point to boot. Chris Morgan, Mark Breeze, Jonathan Griffiths and Jason Howells crossed for tries with Nick Roberts adding a conversion to complete the rout.
Richard Thomas kicked a penalty for the Scarlets and Daniel Evans added a consolation try. However, it was too little too late to prevent the Ospreys from easing into the final. Evans and fellow Scarlet Lloyd Phillips were yellow carded as was Ospreys' Jamie Richards.
The game between the Dragons and Blues, which had been switched from Ebbw Vale to Llantwit Fardre, was subsequently postponed due to the inclement weather.