The 21-year-old has put pen-to-paper on an initial one-year deal with the Pilgrims following some impressive cameos in pre-season.
Deacey has been joined in the United squad by Rossington Main striker Cameron Johnson.
The 18-year-old played and scored against the Pilgrims during the pre-season friendly against Main on July 14 and has been recruited following the completion of an official seven-day approach to the Doncaster-based club.