Prutton began his career at the City Ground, but moved to the Saints in a £2.5million deal in January 2003.
But he has dropped out of the first-team picture at St Mary's, and jumped at the chance to return to his former club.
He said: "Forest are in with a great chance of promotion, which was a big factor in my decision, and it is a very attractive place to play.
"I will have a look and see what is happening at the end of the season, but I don't think I am likely to be back at Southampton."Forest have also signed defender Luke Chambers from Northampton.
Chambers worked under current Forest boss Colin Calderwood, who was in charge at Sixfields before quitting in the summer to take charge at the City Ground.