The former Bridgend, Cardiff and Celtic Warriors star remains Wales' most capped player, having made 100 Welsh appearances over a 12-year spell between 1995 and 2007.
Thomas captained Wales to Grand Slam glory in 2005 and also led the side at the 2007 World Cup in France.
He skippered the British & Irish Lions following injury to Brian O'Driscoll on their 2005 tour of New Zealand and won three Lions Test caps against the All Blacks.
"We would like to thank Gareth for his contribution to the Cardiff Blues over the last three years," said Blues director of rugby Dai Young, himself a former league player with Leeds and Salford.
"He is a larger than life character but is still totally professional and dedicated to his sport. He has kept himself in great shape and has been a great help in bringing on young players.
"His versatility has also been invaluable to us during his time here. It's probably fair to say that Gareth has given his all to Welsh rugby. He requested that he could join the Crusaders as early as possible because their season had just started and we felt it was only fair to let him go with immediate effect to pursue this fresh challenge.
"We would like to wish him the best of luck in this new chapter of his rugby career."
And while he is looking forward to the fresh challenge that his switch will bring, Thomas himself was keen to emphasis how much he has enjoyed his time with the Blues.
"I've loved my time at the Blues and when I joined I felt I was joining a massive family both off and on the field," said Thomas. "Everyone has been great to me: the fans, coaches, the squad and the off field staff, they have all been superb.
"I will have a lot of special memories from my time with them. Bridgend will always be my hometown club but the Blues will always hold a special place in my heart. It's sad to leave, but I'm hugely excited about joining the Crusaders.
"I wanted something that would challenge me and I've always accepted any challenge that has been put in front of me. People may say it's the wrong time in my career, however, I feel that it's right for me.
"I'm not doing it just for the money as it's a vicious league and I want to really test myself. I've never played League before but I imagine the biggest difference will be the instincts."
"You can learn the rules but things that you may do instinctively in union you don't in league and vice versa."