Miller announced on April 10 that he was going to go back to school for his sophomore season.
The NBA early-entry deadline is Sunday. The NCAA had an April 10 deadline to notify schools if a player was returning but it had no bearing on the NBA's early-entry list.
"In the days following his initial announcement to return to Baylor for his sophomore season," head coach Scott Drew said in a release, "we were able to gather more information and Quincy was able to reflect on his decision with his family.
"After receiving the full support of the coaching staff, Quincy thought it was in his and his family's best interest to declare for the 2012 NBA Draft. Baylor Nation thanks Quincy for providing us with an outstanding season to remember and we will miss his outgoing personality."
Miller averaged 10.6 points and 4.9 rebounds in 37 games. He started 35 games for the Bears, who reached the Elite Eight.
Miller's decision underscores what a number of coaches and agents feared when the NCAA moved its deadline to April 10. Players had 19 days to still make a decision for the NBA draft, even after making a public statement on returning to school.