The 19-year-old centre-back has joined Wycombe on a deal until November 12, meaning that he can feature in the Chairboys' next four fixtures.
"Jason's a very highly-rated defender who's had good experience in the Under-21s at Southampton and they feel he's ready to go out and get a taste of what senior football is like at League Two level," Wycombe boss Gareth Ainsworth told the club website.
"Our last two games haven't reached our high standards defensively and he's a new option who adds competition for us, with Anthony Stewart still missing for another couple of months."
McCarthy made his senior debut for Southampton on Boxing Day last year against Crystal Palace in the Premier League. It remains his only senior league appearance.