The club say the 29-year-old's new deal will see him stay "for the 2021-22 season and beyond".
Matavesi has made 27 appearances for Saints since joining them from Championship club Cornish Pirates in 2019, scoring three tries.
"We've been thrilled with his development since he joined the club," said director of rugby Chris Boyd.
"We have an exciting crop of hookers in our squad who are all pushing each other to improve every day, and Sam has taken his opportunity in recent months and performed well as a starter."