The athletic 6-foot-7 Johnson was one of the nation's top incoming freshmen and lived up to the billing, leading the Wildcats with 13.8 points per game and averaging 6.5 rebounds. He was named the Pac-12 freshman of the year and helped lead Arizona to the Elite Eight of the NCAA Tournament for the second straight season.
Johnson was one of two freshmen in the major college basketball conferences to average at least 13.5 points and 6.5 rebounds per game, with Duke's Jahlil Okafor.
He is projected to be a lottery pick in the NBA draft.
Johnson follows teammates Rondae Hollis-Jefferson and Brandon Ashley in declaring early for the draft.