The Saints have been working hard on replacing the stars that have left this summer.
The 25-year-old has arrived from Romanian champions Steaua Bucharest, and is set to take over Dejan Lovren's position in the centre of defence alongside new captain Jose Fonte. The Croatian completed a long protracted move to Liverpool a couple of weeks ago for £20 million.
The giant centre back has often been the partner of Tottenham defender Vlad Chiriches at international level, and for Steaua before Spurs made their move, featuring 12 times for Romania.
Southampton FC (@SouthamptonFC) August 14, 2014
A giant centre back also capable of playing in midfield, Gardin will almost certainly provide a strong aerial presence for the Saints in the penalty area.
It is Ronaldo Koeman's seventh signing of the summer, with the Saints looking to have finally pulled their squad together after the many departures since the end of last season.
The Dutchman was delighted with the signing, telling the Southampton website: "I am very happy to have Florin here.
"He's still a young player so he can develop himself a lot more, and that's part of the ambition of Southampton.
"From the beginning, after Lovren left the club, we paid a lot of attention to Florin.
"Everybody knows the big names and the big players, but it's always more difficult to find one who's not so well known. The information the club has given to me about him has been very good.
"For now and for the future, we have a very good central defender."
The defender, who has signed a four-year-deal after the two clubs agreed a fee said to be in the region of £6 million, said he had fulfilled a dream by moving to the Premier League.
"This is one of the happiest days of my life," Gardos said. "It's the biggest step of my career so far.
"Coming to the Premier League is a very big occasion for me, and Southampton was a team that had a really good last season so this wasn't really a hard decision.
"I've always said that coming to Premier League would be a dream come true, and luckily I've been able to achieve that dream now."