The 33-year-old defender has been at Oakwell for 10 years and made 284 appearances for the Tykes.
Kelvin Etuhu, Emmanuel Frimpong, Liam Lawrence and Paddy McCourt will also be looking for new clubs over the summer along with Jordan Clark, John Cofie, Andrai Jones, Sam Patterson, McCauley Shillito, Erico Sousa, Jordan Tunnicliffe, Iain Turner and Martin Woods.
A statement on Barnsley's website read: "The club would like to thank all players for their efforts throughout their time at Oakwell, especially Bobby Hassell for his 10 years of commitment and service to Barnsley Football Club.
"We wish Bobby and his family all the very best for the future."
Tomasz Cywka, Stephen Dawson and Jim O'Brien have been offered new deals while the club are to open talks with Jacob Mellis.