Foster only joined the Clarets last season after he decided to let his contract run out at Gresty Road.
Barnsley had been keen on the defender for some time and the player did have talks last summer with former manager Andy Ritchie and was offered a two-year deal but he decided to move to Burnley a free transfer.
26-year-old Foster has had a torrid time at Turf Moor and only made seven appearances last seaon and found himself down the pecking order for this season.
This time Barnsley's manager Simon Davey has paid £100k for the defender who has signed a two-year deal at Oakwell.