Matthew Rees was a product of the Pontypridd Under-21 Academy from the 2000–01 season. His nickname at Sardis Road was "Smiler". Rees made one substitute appearance for the side in the 2001–02 Celtic League, along with three appearances as a replacement in the 2001–02 European Challenge Cup. 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.
With the regionalisation of Welsh professional club rugby, Rees joined Celtic Warriors in 2003. Rees made 16 appearances for the Warriors in the 2003–04 Celtic League, as well as one appearance in the 2003–04 Heineken Cup. 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.
Rees spent nine seasons with Scarlets, playing with the team from the 2004–05 season until the end of the 2012–13 campaign. Rees made over 170 appearances for the team and captained the side on a number of occasions.
Rees made his debut for Wales against the USA in 2005.
In June 2009 Rees, Gethin Jenkins and Adam Jones were selected as the British and Irish Lions front row for the 2nd Test against South Africa. This was the first time an all-Welsh front row was selected for a Lions test match since Billy Williams, Bryn Meredith and Courtney Meredith on the 1955 Lions tour.
On 29 May 2010, Rees was voted Rhondda Sport Personality of the year.
He missed the 2011 Rugby World Cup due to a neck injury which meant that Rees would lose the Wales captaincy to Sam Warburton. Wales went on to achieve fourth place.
In 2013 Rees was forced to take time away from the game in November 2013 for treatment for testicular cancer. However he had recovered from his illness and he was able to get two more Welsh international caps again against South African Rugby Union and on 14 June 2014 Rees became the most capped hooker for Wales as he surpassed Garin Jenkins.
