Stanley, 20, confirmed the decision with a post on social media, saying "the past several weeks have given me a lot of time to think. It's really put the past year in perspective. Although the ending to our season was disappointing, I feel blessed to have worn the Duke jersey."
An ACC All-Freshman selection, the 6-foot-6 Stanley is ranked No. 50 on the ESPN Top 100 for the NBA draft. He averaged 12.6 points and 4.9 rebounds, while shooting .474 from the field and .733 at the free throw line in 29 starts for the Blue Devils (25-6 overall, 15-5 ACC).
"It was an absolute joy to coach Cassius this season," Duke coach Mike Krzyzewski said. "I want to congratulate him and his great family on this decision. I've seen Cassius grow both as a player and person here at Duke, and I can't wait to see how his career develops at the next level."
The Blue Devils have had at least one player selected in 32 of 39 drafts under Krzyzewski. He has coached 64 draft picks since taking over at Duke, including 41 first-round selections.
Stanley joins sophomore point guard Tre Jones as early entrees to the NBA draft from Duke.
The Associated Press contributed to this report.