Derek McInnes has left Aberdeen after eight years in charge at Pittodrie.
McInnes took over in March 2013 and steered the Dons to their first trophy in two decades with their League Cup win the following year.
But despite also leading his side to four second-placed finishes, pressure began to mount after a series of poor performances this season.
Aberdeen are currently fourth in the Scottish Premiership table, three points behind third-placed Hibernian but having scored just once in their last 10 games.
The 49-year-old's last match in charge of the club was another frustrating goalless draw at home to Hamilton on Saturday.
Former Rangers midfielder McInnes was linked to a return to the club to replace Pedro Caixinha in 2017, while he also rejected a chance to move to Sunderland.
Aberdeen confirmed the departures of McInnes and his assistant Tony Docherty with immediate effect.
Dons chairman Dave Cormack said in a statement: "During his time in charge Derek raised the bar and delivered a level of consistency not seen at the club for many years, qualifying for Europe each season, and delivering a memorable Scottish League Cup win at Celtic Park in season 2013-14 with 43,000 Dons fans in attendance.
"Clearly, results since the turn of the year have been disappointing and frustrating for everyone. Derek and I reviewed where we were and agreed that this was the right time for him to step down."
McInnes said: "My motivation to deliver success has never wavered and during my eight-year tenure my staff and I have worked hard to elevate expectations and set a minimum standard which should be demanded at a club like Aberdeen FC.
"I would like to thank all the players who have featured in my teams over the years, my current staff and players for their efforts and all the staff behind the scenes for their hard work and endeavour.
"To the supporters for their continuous backing and, of course, the board for their support during my time at the club."