The England international joined Southampton in 2014 from Scottish giants Celtic and has been mainstay at St Mary’s ever since.
Forster’s previous deal was not set to expire until 2021, but the club were keen to give him improved terms amid suggestions that he was wanted elsewhere, and his latest contract runs for a further year.
The 29-year-old insists it was an easy decision to make and has praised the club for continuing to show ambition as they prepare for the upcoming campaign.
Foster told Saints’ official website: “I’m delighted. 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.
“I think it’s a fantastic club to be at, with a really bright future, and the ambition shown at the club is one where they want to move it forward, so for me it was a no-brainer.”