Bartosz Bialkowski has signed a three-year contract with Notts County, following his release from Saints.
The League One side have said that the 24-year-old will be their first-choice keeper for the coming season.
Bialkowski told the Magpies' website: "I'm really excited to be here, to join such a big club as Notts County and hopefully next year we will win promotion.
"I came here because I want to play football and I will do my best to prove that I can be a top keeper. I have not given up on the national team, I still want to play for Poland."
Notts County goalkeeping coach Kevin Pilkington added: "We heard that Bartosz was getting released shortly after the season finished and he was one that we had looked at and one that we think fits the bill.
"He's athletic, tall, kicks the ball well, so hopefully he will go on to do great things for Notts County."
Bialkowski made 42 appearances for Saints between 2006 and 2012. He was let go this summer, after his latest contract at St Mary's expired.