Just hours after the club sacked manager Neil Adkins on Friday, they were heralding the arrival of the 24-year-old centre back for an undisclosed fee from Norwegian top division side Molde.
"It's a dream come true for me. This is where I have always wanted to come and play football. The plan the chairman has for this club is fantastic," Forren, who has five international caps, said on the club's website (www.saintsfc.co.uk).
Forren, had also been linked with moves to Liverpool and Everton but decided to join Southampton.
"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," Forren added.
Southampton lie 15th in the Premier League, just three points above the relegation zone, and will host Everton on Monday. (Writing by Pritha Sarkar, editing by Alison Wildey)
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