Valladolid sack Mendilibar
The head of the B team Onesimo Sanchez has been appointed as his replacement and will be presented on Tuesday.
"The club have decided to look for a drastic solution to a negative run of results that have gone on too long," Valladolid said in a statement on their website.
"Of the last 31 games in La Liga (taking in the final 11 of last season) the side have only won three times."
Mendilibar, 48, steered Valladolid back into the top flight as second division champions in 2007 and led them to 15th and 16th-placed finishes over the last two seasons.
Their 1-1 draw at home to fellow strugglers Almeria on Sunday, however, left them 17th in the standings and one point above the relegation places.