The 24-year-old has signed a three-and-a-half year contract at St Mary’s and moves from Tippeligaen champions Molde FK for an undisclosed fee.
Forren was crowned as the best player in Norway when he won the NISO Golden Ball last November, and has five international caps to his name having become a regular for his country in the latter part of 2012.
The centre-back watched his new team in action during Wednesday night’s 2-2 draw at Chelsea, and was suitably convinced that his future lays on the south coast.
Forren has been allocated squad number 15, and will train with Saints for the first time tomorrow ahead of a possible debut in Monday night’s televised home game against Everton.
Forren said: “It’s a dream come true for me. 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.”