The Warriors star had been in talks with Samoa ever since his decision to return to rugby league late last year, following a stint in Super Rugby and with the All Blacks.
One of New Zealand's most accomplished players with 20 Tests and a Dally M Medal, Tuivasa-Sheck has ultimately decided to honour the nation of his birth and heritage.
His switch comes ahead of Samoa's maiden Test tour of England, meaning the dual-international will not play for New Zealand in this year's Pacific Championships.
It also strengthen's Samoa's desire to be a legitimate force for the 2026 World Cup under coach Ben Gardiner, after reaching the final in 2022.
Jarome Luai, Brian To'o and Stephen Crichton already represent the country, while the likes of Su'a Faalogo are among those coming through.
But in Tuivasa-Sheck they have another out-and-out star capable of drawing in even more talent.
"He's been very complimentary about the opportunity to play for Samoa," Gardiner told AAP.
"He has said if his form merits it he would like to make himself available.
"When I've talked to Roger it's always been about building to the World Cup.
"We're trying to build a team so that when Samoa goes to the World Cup in 2026, everyone has played together and knows the styles and systems we have.
"It gives us an opportunity to take Samoa one better the next time around."
Several big names have also featured in merit sides named by Samoa each month this season, including the likes of Tuivasa-Sheck, Hamiso Tabuai-Fidow, Ronaldo Mulitalo and Payne Haas.
Gardiner is also hopeful Tuivasa-Sheck will not be the last to switch from a tier-one nation this year, with the Penrith assistant having talked to close to 60 players across the NRL in 2024.
"I'd like to hope there would be some other guys who will make the decision, because they want to play for their heritage," Gardiner said.
"We would like to think the camp we put into place last year that was enough to get players talking.
"We wanted to get a real synergy of rugby league high performance culture and religion, the Samoan way.
"The biggest thing that talks around footballers is footballers.
"(If they're saying to each other) 'it's very professional, it's going to be similar or better in that manner to being in New Zealand or the Australian camp,' then I think we a point of difference in our ability to connect the heritage."
Tuivasa-Sheck's move came as the NRL confirmed plans for this year's Pacific Championships, featuring Australia, New Zealand and Tonga in the men's and the Jillaroos, Kiwi Ferns and Papua New Guinea.
The tournament final will be held at CommBank Stadium on the weekend of November 8-10, in what will double as the Kangaroos' first appearance in Sydney since 2017.
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