Samoa secured the final World Cup place through qualification in November 2025 and will face France, Japan and USA in their group in the main event.
Recent World Rugby regulations mean players can now represent another nation three years after their previous appearance for a country.
Sio played his final Test match for Australia against the All Blacks in September 2022 and was eligible for Samoa in 2025.
He will now aim to follow in the footsteps of his father David, who played for Western Samoa in the 1991 World Cup, while Scott’s sister has also represented Samoa.
Sio also recognises the irony of this global tournament being held in Australia.
“That’s funny in itself,” said Sio. “It wasn’t originally in my plans [playing for Samoa] when I went to Exeter.
“I was just grateful for the opportunities I had with the Wallabies and the career I had.
“I was looking forward to going on and establishing myself on this side of the world and playing my career out.”
Sio revealed his family situation had prompted the change.
“Life has a funny way of leading you towards newer things,” said Sio.
“There’s sometimes different stages in your career where you have something different to play for, a different purpose and responsibility.
“I met my lovely wife, who is a proud Samoan Kiwi girl, over in London and we now have a son together.
“That was one of the big things: I wanted to represent them. I play for Samoa and the World Cup is the biggest stage; that would be a great opportunity to do that.
“My dad and sister have played for Samoa, we’re a proud Samoan family, and it’d be nice to represent them back at home.”
Samoa’s rugby authorities and government have been at loggerheads with World Rugby about off-field matters but Sio hopes that can be resolved.
“It’s one of those things where sport and politics are never supposed to mix, so I tend to focus on the sport,” said Sio.
“That’s what I get paid to do, and let the politics take care of itself. In due time, it always tends to sort itself out.
“World rugby needs a strong Samoa, especially heading into a World Cup in Australia where it’s important the Pacific nations play well.
“There’s a lot of Samoans, Tongans, Fijians in New Zealand and Australia that will want to come and watch the nations there.
“You want to be competitive, we don’t want to just go there to take part.”