With the arrival of regional rugby in Wales in 2003, he joined the Celtic Warriors, but their closure a year later led to him signing for the Llanelli Scarlets.

He missed the 2011 Rugby World Cup due to a neck injury, but returned to the team to play twice in another Grand Slam-winning campaign in the 2012 Six Nations Championship.

In the following season, Rees played in three 2002–03 Celtic League games, starting in two, and made three appearances in the 2002–03 European Challenge Cup, two of these coming from the bench.

The Celtic Warriors team was disbanded after its first season due to financial issues however, and Rees moved to Llanelli Scarlets, another one of the recently founded Welsh regional sides.

In November 2015, he was sent off for stamping on Harlequins number 8 Nick Easter in a 2015–16 European Rugby Challenge Cup match just 15 minutes after coming on as a replacement for Kristian Dacey.

[1] In June 2009, Rees was selected in an all-Welsh front row along with Gethin Jenkins and Adam Jones for the British & Irish Lions' second test against South Africa.

[11] Rees was appointed captain of the Welsh side in place of the injured Ryan Jones for the test against Australia at the Millennium Stadium on 6 November 2010[12] and retained the captaincy for the 2011 Six Nations Championship.