Hall played under Saints boss Gordon Strachan during his time at Coventry City and the St Mary's boss claims Hall will act as cover on the left side of Southampton's defence.
Hall made one start for Forest, against Portsmouth on the opening day of the season, before picking up a thigh injury and then losing his place.
He always said that a permanent contract was his main priority but Forest boss Paul Hart made it clear he wasn't in a position to offer him a permanent stay.