The south coast outfit have been active on Thursday afternoon, having already confirmed Shane Long's arrival, and have now captured the 25-year-old Romanian defender
The 25-year-old becomes the club's second signing on a busy Thursday afternoon, having already secured a deal to sign Shane Long from Hull City for a reported £12 million.
Gardos, a Romania international, becomes the Saints seventh summer signing, and has long been targeted as a replacement for Dejan Lovren, who recently moved to Liverpool.
“I am very happy to have Florin here," Southampton boss Ronald Koeman told the club's official website.
“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 [Dejan] 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.”
Gardos, who can play as both a centre-back and a defensive midfielder, says he has realised a dream by moving to St Mary's Stadium, where he will be expected to compete with Jose Fonte, Maya Yoshida and Jose Hooiveld for a starting role.
“This is one of the happiest days of my life. It’s the biggest step of my career so far," enthused Gardos.
“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 had a very good first impression of the club. There’s a very big difference between what I had back home and what is here.
“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.”