The former Sydney Roosters and Canterbury forward returns to rugby league after completing his second stint with the All Blacks for whom he won the Rugby World Cup in 2015 and 2011.

Williams, who lifted NRL premierships with the Roosters in 2013 and the Bulldogs on 2004, will now look to add a Super League title to his glittering resume.

Toronto, who are owned by Canadian-based Australian mining and resources entrepreneur David Argyle, secured promotion to the Super League in September in only their third year of existence.

The signing of the 34-year-old is the biggest in Super League history and Williams said in a statement he was excited about the challenge that lies ahead.

"Toronto Wolfpack has established themselves as a multicultural and economic powerhouse, not only in the city but across the world," Williams said.

"The club has big ambitions and big goals. I want to be part of all this and do all I can to help reach those lofty goals.

"On the field, I'd like to bring high-quality play and do all I can for the betterment of the team.

"Off the field, I'd like to use my experience to mentor the young players and be helpful where I am needed.

"Super League will be a new challenge and one I'm looking forward to. It will be a new experience and one I'm ready to embrace."

Williams also won a Super Rugby title with the Chiefs in 2012 and thanked New Zealand Rugby officials for their support during a career where he was also allowed to box for the country's heavyweight boxing title.

"I have been privileged and blessed to have been given so many opportunities in New Zealand rugby," he said.

"Starting with Canterbury back in 2010, as well as the Crusaders and Chiefs, Counties Manukau and, of course, the Blues and All Blacks.

"I'd like to thank all those fans in New Zealand and around the world who have supported me."

Toronto chairman and CEO Robert Hunter said the signing is a huge shot in the arm for rugby league, not only in Canada but internationally.

"The excitement he will bring to the sport of rugby league in this country will be incredible," Hunter said.

"Having someone of his talent join the Wolfpack will greatly raise the profile of the club, and also help move the game forward globally.

"Sonny is a phenomenal athlete and we believe he is rugby's LeBron James and his addition to our league is comparable to when David Beckham joined LA Galaxy."

Williams will make his first official appearance as a Wolfpack player at a media conference at Arsenal's Emirates Stadium in London next Thursday.

Australian Associated Press