Nafi Tuitavake has signed a new one-year deal at Saints.
The Tongan, who can play at centre or on the wing, has made 29 appearances for Saints since joining the club from French side Narbonne in 2016.
He made his debut in a Champions Cup match at Castres and has gone on to score seven tries in total.
Tuitavake was the only member of last season's squad whose future was still uncertain, but it has now been confirmed that he will be at Franklin's Gardens for the 2018/19 campaign.
“I’m delighted to be staying at Saints for another year, a decision which was very easy for me to make,” said Tuitavake, who has earned eight international caps for Tonga.
“The support I have received from Saints supporters since I have arrived at the club has been very humbling and something I am very grateful for.
“I love pulling on the black, green and gold jersey and running out at the Gardens - when it’s in full voice it’s a special place to play.
“Things are looking very positive for next season and I can’t wait to get back out there again.
"We have a very talented squad with some truly world-class players, and the prospect of working with new coach Chris Boyd next season is hugely exciting.”