The 40-year-old will bolster Banbury's front row for the 2022/23 National League season.
He will also be head coach at the club, joining a management team that includes director of rugby Matt Goode and forwards coach Ian Isham.
Tonga'uiha joins from National 1 side Chinnor RFC Thame, but the prop is best known for his time with Saints, where he made 184 appearances during a seven-year spell.
He played age group rugby for New Zealand, before going on to win 18 caps for his native Tonga, who he represented at the 2007, 2011 and 2015 World Cups.
And Tonga'uiha, who left Saints to join Racing 92 in 2013, can't wait for the next chapter in his rugby career.
"I’ll be a coach firstly, but I still enjoy playing so you may see me running around with the boys when needed," Tonga'uiha said.
"I’m looking forward to developing this young squad and I’d like to pass on any experience I have to the rest of the club.
"There are so many great things at the club, most of all the mini and junior section, so working with their coaches and Matt all ties in well with the club’s vision and mine.”
Director of rugby Goode is very happy to have Tonga'uiha on board.
Goode said: “I’m delighted to formally announce the appointment of Soane as player-coach.
"Banbury now boasts hundreds of Premiership games and dozens of international caps within its coaching team.
"Soane will be a great asset to the club and coaching team.
"What he doesn’t know isn’t worth knowing.
"He is a great guy and very much looking forward to helping Banbury RFC reach its full potential on and off the field."