The 25 year-old centre-back, who has 12 caps for his country, becomes Ronald Koeman’s seventh signing of the summer following the arrival of striker Shane Long at St Mary's.
Southampton boss Koeman told the club's 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 [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 added: “This is one of the happiest days of my life. 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 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.”