Defender Foster, 32, fellow defender Rob Edwards, midfielder Matt Done and John Rooney, young brother of Manchester United's Wayne, will all be released by the club.
Foster joined Barnsley from Crewe in the summer of 2007 and has made over 220 appearances, scoring 10 goals, including a header in the famous FA Cup win at Liverpool in 2008.
Edwards, 30, joined the Tykes from Blackpool in 2011, but has been restricted to 17 first-team appearances and spent time on loan at Fleetwood and Shrewsbury this season.
Done, who followed former boss Keith Hill to Barnsley from Rochdale in the summer of 2011, spent the second half of this season on loan at Hibernian, while Rooney, handed a contract after leaving American ULS side Orlando City in October last year, did not appear for the Tykes' first team.
Barnsley have confirmed goalkeeper Luke Steele, defenders Martin Cranie, Bobby Hassell, Tom Kennedy and Jimmy McNulty, midfielder Tomasz Cywka and striker Jason Scotland will all be offered new contracts.