Barnard agreed the move on Friday but will need clearance from both the Premier League and Football League to play in the Cherries’ fixture against Portsmouth.
The 28-year-old was a key player in the Saints team that won promotion from npower League 1 in 2011, scoring 15 goals in 42 appearances.
Barnard started his career at Tottenham and spent time on loan at Exeter, Crewe, Northampton and Leyton Orient before signing for Southend on a free transfer in January 2008. He was a real hit at Roots Hall and averaged around a goal every other game.
Visit Cherries Player to watch a selection of the new signing’s goals from the 2010/11 season, courtesy of Southampton Football Club.