The 27-year old had a loan spell with Rovers in 2009/10 before signing a permanent contract at Keepmoat where he remained until joining the Saints in January 2012.
The return of Sharp, who spent most of the first half of this season with Reading, has thrilled Rovers boss Paul Dickov who is looking to guide the South Yorkshire club out of the Championship relegation zone.
"I'm delighted to bring back Billy to Doncaster Rovers," Dickov told the club's official website.
"I've been looking to bring in an experienced Championship striker who is capable of winning matches for the club and Billly is that type of player.
"The fact that Billy knows the club so well and has such a fantastic relationship with the fans will hopefully mean he can hit the ground running straight away."
For breaking stories and all the great banter like us on Facebook: facebook.com/tribalfootball