Former All Blacks Charles Piutau and Malakai Fekitoa have also been included in the Tonga squad following changes to World Rugby eligibility rules.
Folau, who is of Tongan descent, had his contract with the Australian Federation terminated in 2019 after posting homophobic messages on social media.
After a brief return to rugby league in 2020 under the jersey of the Catalan Dragons, this evangelical Christian has returned to rugby since last summer in Japan with the Shining Arcs.
Six months ago, a path back to international level opened up for the full-back when World Rugby changed its rules, allowing players who had not played a Test match for three years to change nations and to represent the country where one of their parents or grandparents was born.
Folau, a three-time Wallabies player of the year and Super Rugby's all-time leading try scorer, "is going to bring a lot of experience to the table", Tonga coach Toutai Kefu has told the radio ABC.
"His presence will be one of the most exciting factors that we look forward to," Kefu said.
"He has a whole new set of skills that we haven't had before."
Folau, 33, believes the new eligibility rule could be a game-changer for Pacific Island teams, whose top performers are often drawn to Australia and New Zealand.
"It will only raise the level of world rugby and strengthen these Tier 2 nations," he told AFP in a recent interview.
"It will strengthen Tonga, but also Samoa and Fiji. All these players will be available to represent their nations," said Folau.
Fekitoa, who played 24 Tests for the All Blacks between 2014 and 2017, now plays for Munster in Ireland.
He was born in Tonga and moved to New Zealand at age 17.
Piutau, who played 17 Tests until 2015 for the Blacks and now plays for the Bristol Bears, was born in New Zealand to Tongan parents.
Tonga will face Samoa over two matches on July 10 and 17 in New Zealand, with the winner securing their ticket to the 2023 World Cup.
© 2022 AFP