Andreas Jonker has resigned as Willem II coach, but will remain at the club as technical director.
The club announced the surprise move in a press release, stating that assistant Alfons Groenendijk has taken charge of the team with immediate effect.
The press release said that Jonker "proposed to the board, in the interests of Willem II, that he leave his position as coach".
Willem II got off to a strong start this season, enjoying early victories over Ajax and Feyenoord, but have suffered a slump in form of late, losing four straight games in a winless run that goes back to December.
"This makes it necessary that the coach can fully focus his attention on the first-team selection," the club said.
Jonker will now work solely as technical director, a challenging role in itself as Willem II struggle to balance the books for next season. The club is also developing a new training complex.
"Because of these issues, it was no longer possible to combine the functions of technical director and coach," the statement added.
"The board and directors of Willem II have unanimously decided to promote 44-year-old Fons Groenendijk, up until now an assistant coach at Willem II, to be the new coach of the first team.
"He knows the club and the players, is very ambitious and he has the full trust of the board, directors and players."