The 21-year-old made his first start for the Reds in their Capital One Cup win over Fleetwood at the start of the season, having made his debut as a substitute in May 2011.
Darlow is currently on loan at League One side Walsall, where he is yet to keep his first clean sheet after making three appearances for the Saddlers.
Forest manager Sean O'Driscoll told the club's official website: "He is a goalkeeper who has been impressing for a long time.
"We were both happy for him to go just about anywhere to get some games - he sees that as a progression, to actually be responsible for a team and their results.
"He's doing really well at Walsall and I'm pleased that he's signed a new deal with us."