Saints have completed the signing of Steaua Bucharest centre-half Florin Gardos.
The 25-year-old has agreed a four-year contract with the club, and joins in a deal worth about £6m, becoming the seventh arrival of the summer.
He is also the second capture of the afternoon at St Mary's, following the signing of Hull striker Shane Long in a £12.5m move.
In a club statement, Saints manager Ronald Koeman said: "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 said: "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."
Gardos has enjoyed plenty of success with Steaua, winning the Romanian top flight two seasons in a row, the Romanian Cup in 2011 and the Romanian Supercup in 2013.
He started his career at Concordia Chianja in Romania but his talents were recognised by Steaua, who paid €150,000 for him in 2010.
Since then, he has gone on to play 81 times for the club, scoring three goals.
Gardos had the option to play for Hungary but chose to represent Romania, and has played for them at youth level and full international, where he has so far won 12 caps.