Rotherham have begun the “strenuous task” of replacing Steve Evans after the manager left the Championship club on Monday.
The club said both parties “wished to head in a different direction” and the pair depart New York Stadium after three and a half years in the job.
First-team coach Eric Black has been appointed interim boss as the South Yorkshire club begin their search for a new manager.
A club statement read: “Following a meeting between the chairman and manager (on Monday) morning it was decided Rotherham United Football Club would part company with first-team manager Steve Evans and his assistant Paul Raynor.
“During the said meeting it became clear that both parties at the present time wished to head in a different direction.
“The meeting was at all times positive, constructive and straightforward. It was clear that the chairman and the management team only wanted what they felt was best for Rotherham United Football Club.
“The club would like to place on record our thanks to Steve and Paul for their huge contribution over the last three and a half years, it has been simply an incredible journey.
“Moving on Eric Black will assume first team duties for the time being.
“It has all come as a surprise but Rotherham United must move forward and the search for a new first-team manager will begin immediately. Replacing Steve Evans and Paul Raynor will indeed be a strenuous task for the board.
“Rotherham United Football Club will always welcome Steve and Paul whether as an opposition management team or as supporters of our fantastic football club.”