The 24-year-old has scored 11 tries for the Scarlets and came through the academy system at Stradey Park.
Speaking of the signing, Cardiff Blues Director of Rugby David Young said, "Once Gavin had made the decision to leave the Scarlets to experience a different environment, we were obviously very pleased to hear he was interested in joining the Blues."
"Although he is a young player he has gained a lot of experience during his career at the Scarlets."
"He is a player of high quality and hopefully we can help him fulfill his undoubted rugby potential at the Blues - he will be another great addition to our squad."
Full name Gavin D. Evans
Date of birth July 18, 1984 (age 24)
Height 1.90 m (6 ft 3 in)
Weight 100kg (15 st 10 lb)