The 29-year-old played for the League Two club between 2008 and 2010, scoring 37 goals in 85 games, before leaving to join the Saints.
"It's funny how things work out in football," Barnard told BBC Sport. "I joined on January 25 in 2008, and now I'm back here a day earlier in 2014.
"I've come home to the club where I had a fantastic two years before moving on to Southampton.
"I really enjoyed my time here the first time around and I'm just hoping for more of the same this time around, with just as many goals."
Barnard has not made a first-team appearance for Southampton this season.