The goalkeeper, 34, has penned a new two-year contract to 2026.
McCarthy said: “I’m delighted to extend my stay at a club which means so much to me and my family. To spend the best part of ten years here shows how special a place it is.
“The end of last season will always stay with me. Helping the club get back to where we belong is what we set out to do at the start of the season and I was happy to play my part.
“I really enjoy working with Dean (Thornton) and the goalkeepers we have here. I’m already looking forward to the new season and the test of Premier League football again.”
Saints manager Russell Martin said: “Al stepped up when we needed him most last season and we have a huge amount of respect for him for doing that.
“He’s an integral part of the goalkeeping union Dean has here and knows the values associated with this football club.
“I’m delighted he’s decided to sign, as he brings significant Premier League experience alongside his ability as a goalkeeper, and that will be important next season.”