Sonny William Williams (born 3 August 1985) is a New Zealand rugby union footballer, heavyweight boxer and former rugby league player. He is only the second person to represent New Zealand in rugby union after first playing for the country in rugby league, and is one of only 20 players to have won multiple Rugby World Cups.
In rugby league, Williams has played seven seasons in two spells in the National Rugby League (NRL), with the Canterbury Bulldogs and Sydney Roosters. He has won 12 caps for New Zealand (the Kiwis) and won the RLIF Awards for Rookie of the Year in 2004 and International Player of the Year in 2013.
He first moved to rugby union in 2010 and has since had spells playing for Toulon in France, Canterbury, Counties Manukau, the Crusaders, Chiefs and Blues in New Zealand and Panasonic Wild Knights in Japan. He has won 47 caps for New Zealand (the All Blacks), and was part of the teams that won the 2011 and 2015 Rugby World Cups. He has also played rugby sevens for New Zealand, competing in the 2015–16 World Rugby Sevens Series and the 2016 Olympics.
Williams has boxed professionally seven times, winning all of his heavyweight bouts. He was formerly the New Zealand Professional Boxing Association (NZPBA) Heavyweight Champion and World Boxing Association (WBA) International Heavyweight Champion, but has since been stripped of these titles after failing to respond to challenges.
He is also known as Sonny William Williams.