Alexander-Walker announced his decision on social media on Sunday night and said he would hire an agent.
Walker averaged 16.2 points, 4.1 rebounds and 4.0 assists per game this season for the Hokies, though Duke held him to nine points on 3-of-10 shooting in the Blue Devils' 75-73 win in the Sweet 16.
ESPN ranks Alexander-Walker as the No. 22 player overall -- and the No. 6 shooting guard -- in this year's draft. He was the No. 21 pick in the most recent mock draft on March 19.
Walker's announcement came shortly after Virginia Tech confirmed it was hiring coach Mike Young from Wofford to replace Buzz Williams, who left for Texas A&M last week.
The NBA draft will be June 20.