The 18-year-old centre-back has been a reported target of Tottenham Hotspur and other top Premier League clubs.
However, the new Forest owners have shown their top-flight ambition by tying Lascelles down to a four-year deal.
Chairman Omar Al Hasawi told the club's official website: "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."
Lascelles made his senior debut for Forest in January against Burnley.