The 26-year-old earned 17 test caps for New Zealand between 2013 and 2015, but was overlooked for the last World Cup squad after signing for Ulster.
He is set to become world rugby's highest-paid player when he switches to English club Bristol next season, where older brother and 'Ikale Tahi captain Siale Piutau is already on the books.
Tonga coach Toutai Kefu said the former Tonga Under 20s representative was keen and would add real quality.
"I've spoken to him a few times and his brother as well - he's very determined to play for Tonga so we're very happy about that," he said.
Bristol has a strong Pasifika identity with Manu Samoa internationals Chris Vui, David Lemi, Jack Lam, Tusi Pisi and Alapati Leiua - and former Tongan props Soane Tonga'uiha and Sione Faletau - also among their playing roster.
Kefu said Piutau will be joining a club and coach that had the interests of Pacific Island players at heart.
"Pat Lam really understands our plight and understands that these Pacific Islander players are very keen to play for their country whenever possible."
Piutau is not the only high profile international Tonga is chasing to switch their international allegience.
"There's a winger in France now by the name of Frank Halai, who's had also a cap for the All Blacks. He's a player we're very much interested in and he's a player that's very much interested in coming to play for us," said Kefu.
"There's also some ex Wallabies: Sitaleki Timani and also Anthony Fainga'a - there's a few more as well but that's just the few names that scratch the surface."
Players who have represented one country's national 15-a-side team, next fifteen-a-side team or senior sevens team have the chance to switch allegiance to another nation, provided they had a passport for the second country and have completed a stand down period of three years.
Cooper Vuna played two tests for the Wallabies in 2012 but was able to switch his allegiance last year to represent Tonga at the Olympic sevens qualifying tournament in Monaco, and has since played six tests for the 'Ikale Tahi.
Former New Zealand sevens representative Nafi Tuitivake made his test debut for Tonga in 2016 after turning out for the sevens team at the Oceania Championships while ex Australian sevens representative Ataeli Pakalani debuted for Tonga in July after receiving clearance from World Rugby's Regulations Committee.
The first time a player represents their new union or country must be in an Olympic event, which can include Olympic qualifying tournaments and the 2018/19 World Series, and Toutai Kefu admitted that creates an added complication for Tonga.
"Tonga aren't on the main (World Sevens) series so our tournaments would have to come through the Oceania sevens qualifying tournaments, which at this stage we're not quite sure where they're going to be held at," he said.
"They just have to play two tournaments I think...they just have to be Olympic qualifying tournaments.
"There's a tournament in the World Cup year  that we still need to find out the minor details of that with Oceania [Rugby], but there's a tournament that we may be able to qualify them in around June/July of World Cup year."