The England goalkeeper has committed his future until 2022, just 12 months after signing a previous five-year deal.
Forster joined Saints in 2014 and was the club’s sole ever-present Premier League player last season.
“I’m delighted,” he told the club website. “I have absolutely loved my time here at Southampton so far and when the opportunity came along to extend it I didn’t think twice about it.
Vice-chairman Les Reed said: “We are extremely pleased to have strengthened our commitment with Fraser, who has been an integral part of our team over the past three seasons.”