The 29 year old arrived last summer signing an initial one-year contract and has made four appearances so far in the 2023-24 campaign.
Lumley helped Saints reach the Emirates FA Cup fifth round, keeping clean sheets against Walsall and Watford in the third and fourth rounds respectively.
The shot-stopper has now signed a deal to remain on the south coast until at least the summer of 2025.
Commenting on his new deal, Lumley said: “I’m delighted with it. The first aim is to play and to get the club promoted. But I wanted to achieve the extra year.
“I’m absolutely loving being at the club. Obviously, I would like to be playing in the league, but it’s my duty to sit on the bench and be back up for Gavin (Bazunu), and push Gavin every day and push Alex (McCarthy) every day. Then to improve myself.
“So I’m really pleased with myself and looking forward to what happens at the end of this year and next year.”
Director of football Jason Wilcox added: “We’re delighted to extend Joe’s stay for another year.
“Joe’s a brilliant professional and has added real quality and experience to the goalkeeping group, as well as being an integral part of the dressing room.
“He sets a brilliant example with how he approaches each day and Joe will be valuable as we continue into next season.”
Lumley is yet to make his Championship debut for Southampton but has made 152 appearances in the division across his career.
Having initially joined as third choice keeper, the former Reading and QPR man now regularly sits on the bench ahead of former England international McCarthy.
In joining Southampton, Lumley linked back up with goalkeeper coach Dean Thornton, with whom he had previously worked with at QPR.
Speaking to the Daily Echo earlier this season he explained how important a role Thornton has played in his career.
