The striker moves to the Madejski Stadium on a temporary basis having been left out of Saints' First Team Squad for the 2013/14 season.
Sharp also spent most of last season on loan in the Championship, scoring 11 goals in 40 appearances for Nottingham Forest.
The 27-year-old joined Southampton in January 2012, helping the side seal promotion to the top flight. However, added competition for a starting place saw him move out on loan last September.
He now joins up with the Royals ahead of their home game against Birmingham City on Saturday.