The deal is reportedly worth a $3,231,300 signing bonus, the full value for the No. 10 overall selection.
The Phillies have not confirmed, but they have said they intend on converting Randolph, who played shortstop in high school, to a left fielder.
Randolph, who turned 18 years old on June 2, is considered one of the best all-around bats in the Draft. He hit .526 with seven homers and a .934 slugging percentage during his senior year at Griffin High School in Georgia.
Jamal Collier is a reporter for MLB.com. Follow him on Twitter at @jamalcollier.