Folau had his contract terminated by Rugby Australia in April, with the 30-year-old - a devout Christian - having expressed views which breached the team's code of conduct on his official social media channels.
He posted "hell awaits... drunks, homosexuals, adulterers, liars, fornicators, thieves, atheists and idolaters".
The Wallabies and club side New South Wales Waratahs sacked Folau in May.
However, Folau - who in July opened court proceedings against his former employers - has now announced his return to rugby, albeit in a cross-code switch to League, the sport in which he made his name with NRL sides Melbourne Storm and Brisbane Broncos.
Folau, if fit, will be in Tonga's squad for Tests against the Great Britain Lions in Hamilton on 26 October and the 2 November against Australia in the Oceania Cup.
"I'm so grateful to have the opportunity to represent God, my family, my heritage and the people of Tonga," Folau said.
"I don't want to bring any of my personal matters into this and I am grateful that the RLIF [Rugby League International Federation] have endorsed my availability.
"What I will do now is focus on playing great rugby league and do my talking on the field."
Tonga has also confirmed Folau's brother John - who quit Super Rugby side the Waratahs earlier this year - has been called up.