The 25-year-old Norway international was linked with the likes of Liverpool and Everton before joining Saints for a reported £3.5million.
Forren was Southampton's only signing during the January transfer window and penned a three-and-a-half-year deal at St Mary's.
However, the defender underwhelmed manager Mauricio Pochettino and his staff, failing to make a single first-team appearance for the club.
Now he is on his way back for former club Molde, where he will link up again with manager Ole Gunnar Solskjaer.
Press Association Sport understands Forren's move is permanent and that the club will not lose much money on the deal.
Source: PA
Source: PA