The 24-year-old defender has signed for the Saints from Ole Gunnar Sojskaer's Norwegian league-winning side for an undisclosed fee, although reports have suggested the deal could be valued at £3.5 million.
Forren was previously offered a trial at Liverpool, although the Norwegian declined upon learning of interest from the Saints.
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And Southampton's new signing has revealed his delight at moving to the Premier League: “It’s a dream come true for me," Forren told Southampton's official website. "Southampton is a big club in the Premier League, which is where I have always wanted to come and play football.
“When I was told of the interest from Southampton, straight away I wanted to come and talk to the Chairman. We had a chat and I was very impressed – there was no doubt that this is where I wanted to be.
“The plan the Chairman has for this club is fantastic. There is a good, young squad here and the way they play fits me well, so I hope I can join the team and get some good results.”
Forren began his career at Molde and has made five appearances for the Norwegian national side.