The goalkeeper in the club's longest-serving player and has hailed the impact of chairman Nicola Cortese and Markus Liebherr since his arrival nearly seven years ago
The 36-year-old is Saints' longest-serving player with 283 appearances since arriving in the summer of 2006, with his previous deal set to expire at the end of next season.
Davis has competed for first-team opportunities with Paulo Gazzaniga and Artur Boruc in the Premier League this term, making nine appearances - and the club captain has hailed the impact of chairmen Nicola Cortese and Markus Liebherr since his arrival.
"When I first came to this club, to say that I would have been here for this period of time is something that I would have dreamed about," Davis the club's official website.
"For that dream to have materialised and for me to be feeling good is something I am really, really pleased with.
"I've seen a few things – a few ups and downs – but since Nicola Cortese arrived with Markus Liebherr, the club has been on a journey.
"If those guys hadn't been here then I certainly wouldn't have been either, and it's great that I've been a part of that under Nicola's leadership. That has allowed me to fulfil my dream.
Manager Mauricio Pochettino underlined the impact the goalkeeper has on the squad on and off the pitch, highlighting the experience gained during his 19-year professional career as vital to the club's rise through the divisions.
"As club captain, Kelvin plays an extremely important role at Southampton," Pochettino added.
"His presence in the dressing room is second to none, and his experience has proved to be vitally important to the Club as we look to achieve our aims.
"He has been a massive part of this club during his six-and-a-half years here, and there are few things in football that he hasn’t seen so far in his career.
"Every successful team needs to have characters and people like Kelvin, so I’m delighted that he has committed himself to the club for a further three-and-a-half years."