The 28-year-old Kiwi has agreed a three-year deal with the Tigers, keeping him at the Jungle until the end of the 2011 campaign.
Head coach Terry Matterson was delighted to snare a player of Widders' pedigree: "I've done a fair bit of homework and people I really respect speak very highly of him, he'll be great for us.
"He has played regularly at the top level for eight seasons and I'm sure that his style of play will be a big plus for us and he will be a big favourite with the fans. He is a good character and a really creative strong running back rower with plenty of pace, a good step and he is a proven line-breaker.
"And although we are bringing him in as a back-rower, Dean has also played at centre or five-eighths so he offers us some additional versatility in other positions."
Widders has a wealth of NRL experience, having made 159 first-grade appearances since making his debut for Sydney Roosters in 2000. He spent two years at Sydney and had a five-season stint at Parramatta before joining the Rabbitohs last year.
Widders is the club's second major recruit for 2009 following the capture of St George Illawarra utility Rangi Chase, who was last week named in New Zealand's World Cup squad.