Two tries and an assist by Lee Williams ended a five-match losing streak for the Scarlets and guaranteed them a Heineken Cup spot next season courtesy of a 30-9 win over Cardiff Blues.
Williams, a 22-year-old scrum half playing on the left wing, had shown his abilities as a member of Wales' World Cup-winning Sevens squad, but had never scored in the Magners League before.
This upset - the Blues' first home defeat of 2009 - condemns Newport-Gwent Dragons to be the lowest placed Welsh region and face a European qualifying play-off in Italy.
It also frees five of the Scarlets to travel with Wales to North America, while the four Dragons chosen must miss out to help their region.