Experienced goalkeeper Boruc moved along the south coast in September and has helped Bournemouth to the top of the Championship.
The 34-year-old will now stay with Eddie Howe's side for the remainder of the campaign, although he will not be eligible for Saturday's FA Cup third-round tie at Rotherham United due to the transfer window not re-opening until the morning of the game.
Southampton manager Ronald Koeman acknowledged Boruc's opportunities at St Mary's Stadium were aways likely to be limited following the close-season arrival of Fraser Forster.
"It was not an easy one but when I came I knew it was his last season of contract," said Koeman.
"We wanted to bring in a new goalkeeper, our priority was Fraser Forster and he's shown this season why we changed that.
"That's nothing against Artur because Artur did well for the club but it was a decision for the future of the club and of the team."
Boruc has made 18 appearances for Bournemouth in all competitions, keeping nine clean sheets.
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