Rees has made 56 European appearances - 49 of them in the Heineken Cup - for Pontypridd, Celtic Warriors and the Scarlets.
"Leaving the Scarlets has been a tough decision to make but I feel that moving to the Blues will provide me with a fresh new challenge that I am ready to tackle," he said.
"We've had some great highs during that time in Europe and the Celtic league and I am grateful for the opportunities I have had with the Scarlets as my career has progressed from region to country to the British Lions."
Rees leaves Parc y Scarlets with the club's blessing, Head Coach Simon Easterby declaring: "We respect Matthew's reasons for moving to a new region and wish him all the very best with the Cardiff Blues.
"Matthew has contributed a great deal to the region during his years with us; he has set high standards, made a mark for others to aspire to and been a strong mentor for a number of our younger forwards in recent years pushing their development forward.
"He leaves with our sincere thanks, gratitude as a region and respect for all he has done and achieved in a Scarlets shirt. He has been a key part of our team, has been a true Scarlet in terms of his commitment and imparted experience, knowledge and influence on this team as it developed over the past three or four years - as one of our most senior players.
"He moves on having set a standard which others now aspire to and helped nurture many of up and coming talent forwards in the squad today."