Knox, a 6-9, 245-pound power forward, started 17 games last season at Alabama and averaged 6.3 points and 3.7 rebounds per game.
He is on schedule to earn his undergraduate degree from Alabama this summer.
"It will be an honor to become a member of the Tar Heel family," Knox added. "Go Tar Heels!"
Justin was not available for an interview Monday night and is expected to release a statement on Tuesday.
He chose UNC over Georgia Tech, though also received interest from such schools as Indiana and Oklahoma.