Manager Xisco Munoz has left Watford after less than a year in charge with the team 14th in the Premier League table, the club said on Sunday.
The 41-year-old Spaniard, who replaced Vladimir Ivic in December when Watford were in the second-tier Championship and guided them to automatic promotion, became the first Premier League manager to lose his job this season.
Watford struggled in the top flight, collecting seven points from seven matches after suffering four losses and they are four points above the relegation zone.
"The Board feels recent performances strongly indicate a negative trend at a time when team cohesion should be visibly improving," Watford said in a club statement.
"No further club comment will be available until the imminent announcement of a new Head Coach."
Xisco's last match in charge was Saturday's 1-0 defeat at Leeds United.