The former Tottenham Hotspur and Cardiff defender has signed a one-year loan deal from QPR.
BBC Sport have not mentioned any possibility of a permanent transfer once the loan is concluded, but they have said that there will be no fee for the switch.
The Portuguese international said he felt the arrival of the centre back added necessary depth to Ronald Koeman's side, pointing out Caulker's potential as impressive.
“I think he’s competition, and having a player of his calibre, a player who has represented England, shown a lot of potential, still young, is going to make the other centre backs on your toes and working harder to keep our place in the team,” Fonte is quoted as telling the Echo.
“It’s very important we have this kind of player in the club.
“We want to move forward and for that we need competition in every place.
“I am happy we have signed him and we welcome him to the club.”
Only earlier this week, the Saints captain had stated that central defence was an area of the side that needed dire reinforcements after losing Toby Alderweireld to Spurs and with Florin Gardos' knee injury.
And the 31-year-old hasn't had to wait long for his calls to be answered by the Southampton hierarchy, who have brought in a player with several seasons of Premier League experience despite still only being 23.
The England cap, who was rated one of the best defensive prospects in the country a couple of years ago, has suffered consecutive relegations with both Cardiff and QPR, a record that has tarnished what had been a promising start to his Premier League career at Swansea and then with his sole proper season at Spurs.
Playing as part of what was one of the best defences in the league last season, under a manager who was one of the best defenders of the last 25 years, will only help Caulker become a better player in the coming months. And alongside the unflappable and impressive Fonte, he will surely be grateful to be part of a side that is not regularly exposed like the R's were last year.
QPR conceded more goals than any other side (73) as they returned to the Championship after one season, and Caulker did not cover himself in glory at Loftus Road.
His time at St Mary's will actually be the fourth loan spell of Caulker's career, but with over 200 appearances already for the powerful centre back, it is possible that the London-born player could finally find the right home for his talents at the Saints.