Watford sacked Quique Sanchez Flores, just 85 days after he was re-appointed to the Premier League club for a second spell.
After four league season games, Flores, who succeeded Javi Gracia, won only one of his 10 Premier League matches.
In a 2-1 loss against Southampton on Saturday, Watford suffered their eighth league defeat, which saw them stay at the bottom of the table on eight points and six safety drift.
The decision was made after Sanchez Flores and partner Gino Pozzo met on Sunday with chief executive and chairman Scott Duxbury.
“Quique is a man of great integrity and it was clear how much he wanted to have a positive impact, but ultimately results have dictated our decision.
“The appointment of a new head coach is imminent, and with nearly two-thirds of the season remaining, we will provide all the support necessary to make the coming months successful,” Duxbury said in a club statement.
Arsenal also parted ways with Unai Emery after 18 months in charge of the North London club.