The 20-year-old - who made his NRL debut in 20187 - spurned an approach from Kiwis coach Michael Maguire and confirmed his allegiance to Tonga, NRL.com reported.
Fainu follows a pathway created by four players - Jason Taumalolo, David Fusitu'a, Sio Siua Taukeiaho and Manu Ma'u - who defected from the Kiwis for Tonga on the eve of the 2017 Rugby World Cup tournament.
Manly prop Addin Fonua-Blake, who played for the Kiwis at the World Cup, switched to Tonga in 2018.
Fonua-Blake helped convince Sydney-born Fainu, who has Māori and Tongan heritage, to commit to Tonga.