Rugby League's worst-kept secret finally became official today, with Williams admitting he only left rugby union because of a handshake agreement he made with Roosters Chairman Nick Politis three years ago.
Williams said the decision was "tough" but he would be looking at the season with a "glass half full" attitude.
The 27-year-old said he'd grown to love rugby union after winning the Super Rugby championship this year with the Chiefs and the Rugby World Cup with the All Blacks in 2011.
Williams said returning to league after a five-year absence would be a "huge challenge."
His contract has yet to be ratified by the NRL but the Roosters expected that to be just a formality.
The former Canterbury Bulldogs star fronted his first press conference with the Roosters with his arm in a sling, and said his pectoral injury will take another two to four months to heal.
But he expected to be ready for round one of the NRL season in early March.
He declined to respond to criticism from former Bulldogs teammate Hazem El Masri, who said Williams shouldn't be allowed back in rugby league because of the way he walked out on the Bulldogs in 2008 when he left mid-season to join French rugby club Toulon.
"Haz is entitled to his opinion," was all Williams said.
Williams said he'd met his new Roosters teammates and was looking forward to the season ahead.
"The boys are a great bunch of blokes, a bit like the Chiefs, they're real young," he said.
"I'm looking forward to getting back out there in a game I grew up playing and put my best foot forward.
"It was pretty tough (to come back). It's tough, but me and Nick are good friends, good mates.
"He tried to get me when I was an 18-year-old.
"When I made that (handshake) agreement with him I knew it would always be the case ... glass half full."
Williams also has a boxing career but said he won't fight during the season.
He declined to confirm whether he would be available for the Kiwis' Rugby League World Cup campaign at the end of next season, saying it could depend on his boxing commitments.
Williams was primarily a back-rower in his league days and a centre in rugby union but said he was happy to play whatever position required by coach Trent Robinson.
