Williams, who walked out on the Bulldogs to sign for French rugby union side Toulon in 2008, has agreed a one-year deal with the Tricolours.
He was a World Cup winner with the All Blacks last year and has also carved out a career as professional boxer, winning all five of his heavyweight bouts.
“I am really excited to be making my return with a great club like the Roosters. I have met with Nick and Trent and I am motivated to meet the challenges we have discussed head-on,” Williams, 27, told the Roosters’ official website.
“This isn’t a decision I have made for money – if that was the case, I would be staying with rugby union. I have signed with the club for one season so as to fulfil my pre-existing boxing commitments.
“I have set myself a number of personal goals and challenges I want to achieve as part of this club and want to do all I can to help the Roosters win a premiership.
“There is a great team here that I am looking forward to being a part of and I can’t wait to get started.”
Williams caused outrage when he backed out of his Bulldogs contract with many suggesting he should never be allowed to return to the NRL.
But no-one is ready to deny Williams’ stature as an athelete.
Roosters Head Coach, Trent Robinson, added: “Anybody that has dealt with Sonny knows that he is a professional. He’s a great leader and is to be commended for the way he lives his life both on and off the footy field.
“Sonny is quite a remarkable athlete, an instinctive footballer, and I am really looking forward to working with him as part of our squad.”