Lascelles has been linked with a move away this summer, with Premier League clubs reported to be interested in signing the teenager.
However, the 18-year-old sees his future in Nottingham and told the club's official website: "I am over the moon to have signed a new contract, although to be honest it was a little bit unexpected. I'm just glad to be staying at Forest because that is all I've ever wanted.
"Every day I work hard in training and try my best and want nothing more than to make my mark as a Forest player."
Chairman Omar Al Hasawi added: "We're great believers in the development of exciting young talent and Jamaal fits into that category. Jamaal clearly feels his best hope of continuing to grow and develop as a player is by staying with us and we're delighted he has signed a new long-term deal."
A product of the Forest youth system, Lascelles made his first-team debut against Burnley in January and gained further valuable experience while on loan with Stevenage in the closing stages of last season.
