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The All Blacks star will be fighting against an undisclosed opponent in Sydney on December 1, in a charity event called 'Banger in the Hanger'.
His opponent will be revealed next week.
The bout will be a quick turnaround for Williams, just days after the All Blacks' last game of the season against Italy in Rome on November 24.
"I'm looking forward to getting back in the ring again. I know this will be a hard fight for me after being out of the ring for three years. Some people will say I'll be rusty. But I can't wait! I know I'll put on a good show and the cause is a great one. It feels good to be giving something back," Williams said.
"The winner of the fight will be known as the WORLD HEAVYWEIGHT CHAMPION OF THE HOMELESS & NEEDY as proceeds of the night will go to helping the homeless in Australia and New Zealand," a press release said.
All Blacks coach Steve Hansen has supported Williams' decision to step back in the ring.
With the Rugby World Cup being so close, many have questioned Williams' dedication to the All Blacks but Hansen wasn't worried about the planned boxing excursion.
"Look, from my understanding I think it's a very, very low-key fight and it's about raising money for the homeless in Sydney and for the Auckland City Mission," Hansen said.
"He is pretty confident, I've spoken to Khoder [Williams' agent Khoder Nasser], and Khoder's confident that what's going to take place won't be of hindrance to him, so you've got to back their judgement and from what I understand it shouldn't be too much of a task."
Injury hit Williams, who is still recovering from a shoulder injury he copped during the All Blacks' 49-14 win over France in June, has also had issues with concussions in the past.
When asked if 33-year-old Williams will have to juggle his All Blacks duties getting ready for the fight, Hansen said: "No, I think he's just going to get off the plane and do it. That's the sort of athlete he is.
"We've got guys boxing all the time, as part of their training. Again, my understanding of the fight is that it's more of an exhibition fight I think than a go down and knock yourself out fight.
"You're probably better off having a good yarn to him about it, because he'll be able to fill you in on all the details. But I don't have any problem with him doing it."
Williams has urged former NRL rival Paul Gallen to be patient, as talk of a much anticipated fight between the two gathers momentum.
Gallen has expressed his frustration with being overlooked as Williams' next opponent, claiming he was being dodged by the All Blacks star.
Speaking to the Sydney Morning Herald, Williams said the opportunity to raise funds for the homeless via the Exodus Foundation in Australia and the Auckland City Mission in New Zealand was too good to pass up.
"Tell my good friend Paul, he can just relax and we will get there eventually," SBW told the Sydney Morning Herald. "He keeps calling me out by attacking me and questioning my manhood. That kind of talk, I'm past that stage in my life.
"I've been involved in sports for long enough to understand that the bravado, the macho talk is just rubbish. That's just water off a duck's back to me, so to speak. That's what I'd say about that situation brother."
When asked if he was keen to take on Gallen in the future he said: "Yeah, of course. When it comes to my good friend Paul, he probably needs to relax a bit.
"If it happens, it happens. This great opportunity came up for such a great cause. Big Paul had to take a back seat."