The 26-year-old prop caught the eye of a host of scouts after playing in all four of the Tomahawks' World Cup matches, including their 24-16 win over Wales in Wrexham, as they set up a quarter-final with Australia.
Crusaders coach Clive Griffiths said: "I'm absolutely delighted that Mark has agreed to sign with us. I believe he'll be a really important player for us next year.
"Considering the amount of attention he was receiving from Super League clubs and also an enquiry from at least one NRL outfit, the fact he has decided to come to Crusaders is a major coup.
"I think the timing was just perfect, as had he come to the attention of the top coaches a little earlier in the year he'd have undoubtedly been snapped up."
Offerdahl is a product of the Manly Sea Eagles club and last season played in the French domestic competition for Carcassonne before taking up a short-term contract in rugby union in America.
He will team up at the Racecourse Ground with with his old Carcassonne team-mate Stuart Reardon, who was instrumental in convincing Offerdahl to sign for the Wrexham club.
Chief executive Jamie Thomas said: "We've really got to thank Stuart quite a bit for getting in Mark's ear and telling him what a great place to be Crusaders is. I've no doubt he'll fit in well with the boys and we can look forward to the year ahead."
- Sports & Recreation