Millwall head coach Neil Harris will depart the club after Saturday’s match against Middlesbrough.
Currently 11th in the Championship, Millwall has not won any of their last four matches.
Harris rejoined the club in February for a second spell, signing a contract until June 2025.
The 47-year-old admitted that leaving Millwall was a difficult decision, as he is the club’s all-time leading goalscorer and previously managed the team from 2015 to 2019.
“Everyone knows this club is more than a job to me, it’s my club, and always will be,” Harris said.
“I always said when I returned that I would leave when the time felt right, and now is that time.”
Millwall were one point above the relegation zone when Harris took over in February last season and he went on to steer them to 13th in the Championship.
They have won six of their 18 games so far this season and are six points off the play-off places, with defeat at home by Coventry on Saturday their first loss in 10 games – although they did not register a single shot on target.
They face Sheffield United at home on Wednesday in what will be Harris’ final time in The Den dugout, before the trip to the Riverside on Saturday.
A club statement said recruitment is already under way for Harris’ successor, with interim management plans to be announced after the weekend.
“A comprehensive recruitment process was already under way for a new head coach,” said director of football Steve Gallen.
“The direction we as a club want to head in is clear and the next head coach will play a pivotal role in helping us execute our strategy.”
Chairman James Berylson said he accepted Harris’ decision after discussions together in recent days.
“As we look forward, the appointment of our next head coach will align with the clear strategy and vision I have previously laid out for the future of Millwall Football Club, both on and off the pitch,” he said.