Danny Wilson has been sacked as Barnsley manager with coaches Paul Heckingbottom and Mark Burton put in caretaker charge.
Assistant manager Chris Hutchings has also left the club, who are an 'unacceptable' 17th in League One according to a club statement.
Wilson took the Reds to the Premier League in 1997 and was brought back in December 2013 with the club fighting relegation from the Championship. The most experienced manager in the Football League could not keep them up last season and lost 31 of his 63 games in his second spell.
A Barnsley FC statement said: "The Club would like to place on record its thanks for Danny's work-ethic and determination during his time with the Club and the support Chris provided Danny with since he joined the Club in March 2014.
"Danny joined the Club at a very difficult time which was a testament to his character and affection for the Club – all the while demonstrating the utmost professionalism during his time with the Club. Danny will always be welcome back at Oakwell.
"The success he brought during his initial period with the Club is unrivalled and the Club will be forever grateful for his achievements.However, recent performances and results have been concerning, considering the Club currently boasts the fifth highest wage bill in League One in addition to a fourth highest equity donation by our owner.
"Meanwhile, our current League position of 17th is in stark contrast to an average home gate of sixth-best in the division. Whilst the Club acknowledges there has been a large turnover of players, the league position is still unacceptable considering all of the above."
