Casty will join the Roosters from Super Leagues Catalans Dragons, for whom he has played his entire top-level career since making his debut in 2006.
Viewed as a leader on the field, Casty is recognised as one of the most dangerous forwards in the Super League, known for his strong and powerful tackling style. He was named in the 2012 Super League Dream Team.
The utility forward has made seven appearances for France to date, winning their Player of the Year award at the 2011 RLIF Awards.
Casty will join the Sydney Roosters at the conclusion of his World Cup commitments later this year.
Click here to see some highlights of our newest signing in action.