The 23-year-old has agreed a four-year contract with Saints and moves from French side Olympique Lyonnais for an undisclosed fee.
Lovren's future has been the subject of much speculation in recent months, with a number of top European sides being credited with an interest in acquiring his services.
The Croatian international has decided that his footballing future lies at St Mary's, putting pen-to-paper this evening as the Club's Executive Chairman Nicola Cortese and First Team Manager Mauricio Pochettino watched on.
Lovren will now link up with his new teammates when the First Team returns to Staplewood for pre-season training at the start of July.
"I’ve spoken a lot with the Chairman and the coach, and this is a great project," Lovren said shortly after completing his move.
"They told me a lot of positive things, and I know the history of the Club. It’s a big club and I’m very happy that I can be a part of it.
"After three-and-a-half years in France, it was a dream to play in the Premier League. Now the dream has come true.
"I have it in my mind to do something big, and I see the same thing in the Club.
"We are a young team, and I am a young player also so I hope I will stay here for a long time."
Pochettino spoke highly of Lovren, adding that he was pleased to have completed his first signing of the summer with more than a fortnight left until pre-season gets underway.
"We are very happy because Dejan is a very good player with good potential," enthused the manager.
"In pre-season it’s important to have all players working together, so [Lovren’s signing] is a very good job by the Chairman and all of the staff at the Club."
In Profile – Dejan Lovren
Lovren was born on 5th July 1989 in the town of Zenica.
He began his career in the youth teams of NK Karlovac where, in 2004, he was spotted by Croatian giants Dinamo Zagreb after impressing their academy coach Zeljko Adzic while playing against them.
He soon joined Dinamo and moved into their youth sides, and was included in a senior squad for the first time shortly before his 17th birthday.
By that point, Lovren was already a regular for his country at youth level – having made his debut for Croatia’s Under-17s aged just 15, he played for them 18 times and scored twice.
He rapidly ascended through the ranks, playing for the Under-18s, Under-19s and Under-20s before becoming an Under-21 international in 2007.
Dejan was loaned to newly-promoted NK Inter Zaprešić at the start of the 2007/08 campaign. During his first full season as a professional, he started 29 of a possible 33 league games as Zaprešić finished 11th in the top tier of the Croatian league.
Happy with his development during his year in Zaprešić, Dinamo – who had just won their second successive title – handed Lovren his opportunity to impress shortly after the start of the 2008/09 season.
Still aged only 19, the defender became a regular starter for his parent club, scoring his first goal in their colours in a UEFA Cup draw away at Sparta Prague.
He netted his second for Dinamo just three days later, in a 1-0 home success over NK Osijek, and went on to make a total of 22 league appearances plus a further eight in Europe as his club secured their third back-to-back title.
In August 2009 Lovren was called into the full Croatia squad for the first time but did not make it off the bench during a 2010 FIFA World Cup qualifier against Belarus. His debut would arrive two months later, when he appeared as a second-half substitute in a friendly against Qatar.
In 2009/10, Dejan continued to play regularly for Dinamo in the 1. HNL and the UEFA Europa League as interest from outside of Croatia grew.
In January 2010, after making 66 appearances for Dinamo, Lovren moved to French Ligue 1 side Olympique Lyonnais, making his debut later that month when he started a Coupe de France match away at AS Monaco.
He went on to make eight Ligue 1 appearances in the second half of the 2009/10 season, while continuing to pick up caps for Croatia.
Having turned 21 in the summer of 2010, Lovren came to prominence in his first season at the Stade de Gerland, playing regularly in Ligue 1 and the UEFA Champions League.
His first goal for Lyon came in a 4-3 loss away at SL Benfica in the Champions League as Les Gones reached the last 16 before going out to Real Madrid.
In September 2011 Lovren scored his first international with a towering header in a 3-1 win over Malta that put Croatia en route to qualifying for the 2012 UEFA European Championships.
He would go on to be named in Slaven Bilic’s final 23-man squad for the finals in Poland and Ukraine, but unfortunately had to withdraw before the tournament began.
In 2012/13, Lovren played his part as Lyon finished third in Ligue 1 to seal their place in next season’s Champions League qualifying round. During the season he also scored his second international goal with a stunning strike in a 2014 FIFA World Cup qualifier against Wales in March of this year.
Lovren arrives at Southampton having made more than 70 appearances during his two-and-a-half years with Lyon and with 17 international caps to his name.