The Wildcats confirmed Hagans will forgo his remaining college eligibility.
The former five-star recruit averaged 11.5 points, 6.4 assists, and 3.9 rebounds in 30 games this past season, helping Kentucky finish atop the SEC standings.
"It's been my No. 1 goal since I first picked up a ball to take this step, and I know my time is now," Hagans said about declaring. "It's so hard for me to say goodbye to the best fans in the world because these last two years have been some of the best of my life."
Wildcats head coach John Calipari had encouraging words for the point guard.
"Ashton is ready for this," Calipari said. "From where he was when he got here two seasons ago to where he is now isn't even close. He's a completely different player and he's built his skills and his confidence to take that next step.
"We know what Ashton can do to change the game defensively and how much better he got offensively, but what's going to separate him is his competitiveness. That kid is a winner. His competitive spirit drives him and is why I love him. If I'm going to battle, I'm taking Ashton."
ESPN ranked Hagans as the 49th-best available player in the draft.