Former Netherlands defender Frank de Boer is favourite to replace Martinez, according to several British bookmakers, after he left Ajax Amsterdam earlier in the day.
Everton confirmed media reports that the Spaniard had left, with three years remaining on his contract.
"The chairman and the board of directors would like to place on record their thanks for the dedication and commitment Roberto has shown during his three seasons at the club," Everton said in a statement.
"Roberto has been a great ambassador for the club, conducting himself at all times with the utmost integrity and dignity."
Everton reached the semi-finals of both domestic cups this season but it is their disappointing league form that has cost Martinez his job.
After a bright start, the Goodison Park team have tailed off alarmingly and are 12th going into the final weekend of the season having won once in their last 10 league games.
Fans have become increasingly disgruntled, especially as the squad is one of the best the club have assembled for many seasons with the likes of Belgium striker Romelu Lukaku, England midfielder Ross Barkley and former Barcelona midfielder Gerard Deulofeu.
LIVERPOOL DRUBBING
Since a 4-0 rout by local rivals Liverpool last month, banners have been displayed at games calling for Martinez to go.
The 42-year-old has a reputation for producing sides that are easy on the eye but Everton have struggled defensively this season, conceding 55 goals in the league ahead of Sunday's finale at home to Norwich City.
When Martinez first replaced long-serving manager David Moyes in 2013, the former Swansea City and Wigan Athletic bossimpressed as Everton finished fifth with 72 points, a total usually good enough to secure a top-four spot.
Last season they reached the last 16 of the Europa League but regressed in the Premier League, finishing 11th with 47 points.
The downward trend has continued in the league this term despite around 30 million pounds spent on Ramiro Funes Mori, Oumar Niasse, Aaron Lennon and Deulofeu.
They have lost eight of their 18 home league games, the worst record in the club's history.
(Editing by Tony Jimenez)
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