The defender was expected to complete a switch to the South-Coast side after deciding to reject a trial at Premier League rivals Liverpool.
"It's a dream come true for me," Forren told the club'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, who will wear the number 15 shirt, could make his debut against Everton on Monday evening.