Watford fired Xisco Munoz after 10 months on the job.
The 41-year-old Spaniard helped win the Premier League promotion last season after joining in December 2020.
Watford are 14th with seven points from seven games and Munoz’s last game was a 1-0 win on Saturday against Leeds United.
“The board believes that the recent performances show a negative trend at a time when team cohesion should be seen to improve,” the club said.
“The Hornets will always thank Xisco for his role in securing promotion from last season and wish him all the best in his future football career.
“No further explanation of the club will be available until the announcement of the new head coach.”
Watford have had five managers in two years and there is no indication as to who will replace Munoz.
The club, which was also knocked out of the Carabao Cup by Stoke in September, will face Liverpool on October 16 in a league match.