The 35-year-old oversaw only three wins in 15 games in charge and ended the campaign with five consecutive defeats as Watford finished 16th in the second tier.
"Watford FC have this morning parted company with head coach Ed Still," a statement said.
"First-team coach Karim Belhocine has also left the club. We wish them all the best in their future endeavours."
The Hornets began the campaign under Paulo Pezzolano.
He was replaced by Javi Gracia in October as the Spaniard took charge at Vicarage Road for a second time before resigning in February.
Watford are now seeking a 24th permanent head coach since 2012, and their 16th in the last seven years.