Former England Under-21 international McCarthy has become Southampton’s third signing of the summer after falling down the pecking order at Selhurst Park following the arrival of Steve Mandanda.
The 26-year-old is likely to act as deputy to England stopper Fraser Forster, but the former Reading man is confident he will be given a chance to impress new boss Claude Puel.
“I’m over the moon to join a club like this. The club is known for producing young players and they’ve had successful seasons,” he said.
“There’s the Europa League to look forward to this season, so there’s going to be a lot of games.
“With the stature of the club, I’m honoured to be here and I’m looking forward to getting working with everyone.”
And Saints Director of Football Les Reed lauded the credentials of the club’s new arrival.
“Once again, I am delighted to welcome another player to the squad at Southampton. We now have two quality English goalkeepers who have both represented the country at International level,” he said.
“I am confident that Fraser will soon be England’s number one, and in Alex we have a young player who has already had a taste of the England senior squad, as well as excellent experience at Under-21 level.
“Alex is ambitious and will be pushing for a starting place every time he gets the opportunity, which can only be good for Fraser and a shining example for Harry Lewis and the younger keepers below.
“Working with Dave Watson, our keepers can only get better and, who knows, we could have two Saints stoppers at the World Cup in two years’ time.”