The 32-year-old has decided to end his time with the Scarlets and begin a new "challenge" by signing a two-year contract with the Blues.
"Leaving the Scarlets has been a tough decision to make after being there for so many years," Rees told Cardiff's official website.
"The Scarlets is where I established myself as a player however I feel that this is the right decision at this point in my career.
"For me personally, I feel that moving to the Blues will provide me with a fresh, new challenge that I am ready to tackle."
Rees has so far earned 57 caps for Wales.