Railey is the fifth straight high school player the D-backs have drafted.
According to his scouting report, Railey may need to move to a corner outfield spot because he is an "average" runner, but he projects to be a serviceable defender.
"Gives us a chance to add a left-handed bat with kind of a hit-speed combo," D-backs scouting director Ray Montgomery said of Railey, who was scouted by Luke Wren.
As a high school senior, Railey hit .407 (35-for-86) with four home runs and 10 doubles.
The Draft concludes on Saturday, with exclusive coverage of Rounds 11-40 beginning on MLB.com at 10 a.m. MST.
Steve Gilbert is a reporter for MLB.com. Read his blog, Inside the D-backs, and follow him on Twitter @SteveGilbertMLB. Adam Lichtenstein is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.