Thornton is expected to take over the spot vacated by Tyus Jones's departure to the NBA. Jones averaged 11.5 points and 5.7 rebounds per game last season.
"Once Tyus declared, Duke came after me hard," Thornton told ESPN.com.
Thornton was originally set to graduate from high school in 2016 but will now enroll at Duke in time to play next season.
Four starters from Duke's national championship team will not be back next season. Jones, Jahlil Okafor and Justise Winslow all declared for the NBA draft after completing their freshman seasons, and senior Quinn Cook will graduate.
- Dan Gartland