Thomas, 36, is in his first full season with the Welsh Super League side having joined part-way through last season from rugby union.
He said: “I am really excited to be signing for another year at the Crusaders.
“To stay on with the Crusaders, who are a Welsh rugby team with big ambitions is something I am proud of, and I am really pleased to be a part of those ambitions and to do my part to help the club and its profile grow.
“I’ve had a great two seasons with the Crusaders so far, it has enabled me to play and captain Wales again and I can’t wait for next year.”
Thomas has scored seven tries in 26 appearances for the club since his arrival in March 2010, and was part of the Wales team that qualified for the end of season Four Nations by winning last year’s European Cup.
Crusaders Chief Executive Rod Findlay added: “This is great news for the Crusaders.
“We spoke at great length with Gareth about signing a new contract with the club. He has been a model professional since joining the club and his performances, particularly this season, have been fantastic.
“He has helped improve the profile of not only the club, but Rugby League in general. But first and foremost, he is a quality rugby player who we’re very pleased to have on board until at least the end of the 2012 season. Gareth still has a lot to give as a player and we’re pleased that he has decided to do that with the Crusaders.