He will move to the Welsh capital at the end of the season to join what looks likely to be a new-look Blues as coach Phil Davies reshapes his squad.
"I feel that moving to the Blues will provide me with a fresh, new challenge that I am ready to tackle," said the 32-year-old," he said.
"Leaving the Scarlets has been a tough decision to make after being there for so many years. It's where I established myself as a player.
"However, I feel that this is the right decision at this point in my career. Joining the Blues at this stage will, I hope, freshen things up for me as I've still got ambitions to play for Wales."
Rees captained Wales until the 2011 World Cup, which he had to miss because of injury.
He started for Wales in last autumn's matches against New Zealand and Australia and will be one of the top contenders for a place in the Lions squad to tour Australia this summer.
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
A cool looking all black rugby cleat with the high performance adidas is known for. Get in the Gear!
Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
$29.99
The All Blacks Performance t-shirt is black with hints of blue from the training jersey. Very Cool.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.
