"This is one of the happiest days of my life. It's the biggest step of my career so far," Gardos told the club's official website.
"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."
Manager Ronald Koeman added:,"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."