The New Zealand centre, who set up a crucial try within moments of coming on at half-time during the final, explained that the boy, identified on social media as 14-year-old Charlie Lines, had been tackled by a security guard after trying to run on the pitch.
Williams explained afterwards:
A young fella snuck on the field somehow but when he was coming up to give me a hug, he got smoked by a security guard, full-on tackled him.
He was only eight and the other fella was a full-on man, so it looked like he would break his ribs or something.
The moment probably just got the better of him but he was just so excited to get on the field with the All Blacks. I thought I'd make it a night to remember for him, rather than my medal being hanging up at home or something like that.
But Saturday's act of generosity is not the first time Williams has showcased his selflessness during this World Cup.
Following the All Blacks' victory over South Africa in the semi-finals, Williams was seen consoling devastated Springbok Jesse Kriel who was hunched over by the goalposts.
Picture: GettyPicture: Getty
It shows the kind of player and the kind of person he is, and shows what rugby is about.
The week before, Williams, who converted to Islam in 2008, also offered two tickets for the semi-final to a Syrian refugee agency based in London.