Tonga meet Fiji and Japan on July 22 and 30 respectively, before the World Cup in France in September.
Coleman - whose late father Pau'u Afeaki was Tonga skipper - played 38 times for Australia before signing with London Irish in 2019, captaining the English Premiership side last season.
The 31-year-old has now formalised a switch to the Pacific island nation after being approached by former Wallabies No.8 and current Tonga coach Toutai Kefu.
World Rugby eligibility rules allow a capped player to switch allegiance to another country for which they are eligible following a three-year cooling-off period.
"He's very keen to represent the Tongan side of his heritage and experience a World Cup again. It's a connection he feels strongly about," Kefu said previously.
Coleman will again link up with Folau, an outspoken Christian who won 73 caps for the Wallabies before being sacked in 2019 after a social media post about gay people.
The talented back has since switched allegiance to Tonga, who are in a tough World Cup group B that also includes Ireland, defending champions South Africa, Scotland and Romania.
