Hasselbaink is reunited with Reds boss Steve McClaren, who managed the Dutchman at Middlesbrough as part of the team which reached the UEFA Cup final in 2006.
Since retiring from professional football in 2008, Hasselbaink turned his attentions to coaching and has been attaining his UEFA 'A' Licence.
The 39-year-old was invited to join Forest for pre-season training and, speaking after his appointment, he told the club’s official website, www.nottinghamforest.co.uk: “I couldn’t be happier working with Steve at a big club like this and I was delighted to get the opportunity.”
He added: “Steve knows how to talk to players, knows what they want and he makes them feel comfortable. Above all, he is a football man and likes to play the game in a proper way.
“His attention to detail is excellent and one of his strengths is making the players aware of that as well.”