The 24-year-old joins brothers Latu and Samuela Fainu, both contracted through 2027, in committing past the 2026 NRL campaign that begins next month.
The trio of Manly juniors loom as integral to the long-time strugglers' rebuild under Benji Marshall, with Sione a regular in the middle rotation.
Latu Fainu is considered a long-term halfback option but is currently sidelined with a shoulder injury, while Samuela will start the season on an edge.
Eldest Fainu brother Sione can also play second row in a pinch and appeared in all 24 games of the Tigers' improved 2025 campaign, his breakout season.
"The Tigers gave me a chance to make my NRL debut and start my career, I'm grateful to Benji and everyone who has supported me," Fainu said.
"I've learnt so much being part of this group and playing alongside some amazing players.
"The chance to share this experience with my brothers is really special and I'm looking forward to this season."
Jeral Skelton is the only other out-and-out first-grader whose Tigers contract expires after this season.
But the club is also waiting for fullback Jahream Bula and playmaker Jarome Luai to exercise contract options that will confirm they're staying past this year.
Elsewhere, local product Thomas Cant has signed a two-year contract extension to remain with Newcastle until at least the end of 2028.
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