Saints manager Gordan Strachan swooped for Prutton late on Thursday night and finalised the switch on transfer deadline day.
The England under-21 star however, did not join the Saints in time to be included in their squad for the Premiership game with Manchester Utd on Saturday and will be cup-tied for their FA Cup clash against Millwall next week.
Prutton was linked with Newcastle in January last year before they decided to buy Jermaine Jenas instead.
It is believed Forest cashed in following his refusal to sign a new contract which was due to end next summer.