Thomas, 35, makes the code switch after an agreeing an 18-month deal with the Super League club.
He said: “I’m really excited by the ambition of the Crusaders.
“This move was not about money but about a chance of a fresh challenge on the field and an opportunity to promote rugby in north Wales off the field.
“Playing for someone of the experience and reputation of (coaches) Brian Noble and Iestyn Harris really excites me.”
Thomas is the only player to win 100 caps for Wales and captained them to their first Grand Slam in 27 years in 2005. He moves from Cardiff Blues rugby union club, and has also played for Bridgend, Celtic Warriors and Toulouse.
Crusaders chairman Ian Roberts said: “This is a truly monumental day in the Crusaders’ history.