The goalkeeper has left Crystal Palace after just one season, where he made 7 Premier League appearances, failing to keep a clean sheet in each of those matches.
McCarthy has only played in 10 Premier League games since the end of the 2013-14 season, with spells at QPR and Palace unable to lead to much game time.
But the 26-year-old is excited about joining the Saints, and said he leapt at the opportunity to move to St Mary’s:
“I played with a few of the lads here already, so I spoke to a couple of them and they had obviously nothing but good words to say about the place, so it was an easy choice for me.”
McCarthy also sighted the club’s coming UEFA Europa League campaign as a bonus, with Southampton set to discover their group stage opponent son August 26th.
The keeper is set to give some competition to Fraser Forster, who missed out on 20 league games last season because of injuries.