The 21-year-old, who is currently on loan at League One side Walsall, made his debut for Forest in May 2011.
"He is a goalkeeper who has been impressing for a long time," manager Sean O'Driscoll told the club's official website.
"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."
Darlow has failed to keep a clean sheet in three appearances for Walsall so far this season.