The Roosters got the former All Black's complicated one-year contract ratified by the NRL just before the Friday deadline expired.
Williams faces a race against time to recover from knee and pectoral muscle injuries to play next Thursday's potential sell-out at the Allianz Stadium in what could be his first rugby league game since 2008.
"The Roosters have worked closely with the NRL regarding this matter and we were comfortable with the proceedings," Roosters chief operating officer Brian Canavan said on the club website.
"We now look forward to continuing our preparations for the season-opener against Souths, and getting on with what will hopefully be a memorable season of football."
Williams missed Rooster trial matches after arriving late following his boxing bout with Francois Botha in Brisbane and his knee was heavily strapped at Friday's training run.
Williams beat South African Botha on points last month in a controversial fight which was reduced from 12 rounds to 10 at the last minute.
"All the boys are really enjoying training with him and really looking forward to playing with him," Roosters prop Martin Kennedy was quoted as saying by the Australian Associated Press.
"He's obviously got that aura around him as an athlete. He's just one of those guys you want in your side."