On 30 August, Williams was named in New Zealand's 31-man squad for the 2015 Rugby World Cup. On 20 September, he played off the bench in New Zealand's opening game victory against Argentina. Four days later, Williams played in New Zealand's first-ever test match against Namibia. On 2 October, Williams played in New Zealand's first-ever test match against Georgia. The following week, he scored a try in New Zealand's final pool stage match against Tonga. On 17 October, Williams was a part of New Zealand's record-setting quarter-final victory against France, before playing in the semi-final victory against South Africa. He went on to play in New Zealand's victory against Australia in the final, before gifting his winner's medal to a 14-year old fan who had been tackled by a security guard during the lap of honour; however, he was presented with a second medal at the World Rugby Awards the following day. His performance during the tournament, along with those of his midfield partners, was praised by coach Steve Hansen: "Not only have we lost Ma'a and Conrad we've also lost Sonny. They were the three guys that set the World Cup on fire and allowed us to have a one, two, three punch."