Fiorentina have dropped to tenth in the Serie A after winning only one out of their last 11 matches in all competitions and suffered a shock 1-0 defeat at home by next-to-bottom Frosinone on Sunday.
However, they have reached the semi-finals of the Coppa Italia and drew their first leg 3-3 at home to Atalanta.
On Monday, the club issued a statement strongly criticising the team's recent performances but said it wanted Pioli, in charge for one year and 10 months, to lead the recovery.
"The owners are absolutely not prepared to accept what has been happening for the last few months," it said
"The team must go back to being what it was: competitive, courageous and proud of the shirt they are wearing."
The club then asked Pioli to "handle this situation with the competence and seriousness he showed in the first half of the championship."
On Tuesday, Fiorentina said in a short statement that it had received Pioli's resignation while the coach told the ANSA news agency that he had been "reluctantly forced to leave."
"I have always assumed my responsibilities, I have always guaranteed professionalism, respect and maximum commitment in my work," he said.
"Today, I reluctantly see myself forced to leave, resigning, as my professional and above all human abilities have been questioned."
Pioli, coaching his 12th Italian team, had been nurturing a young team around players such as forwards Giovanni Simeone (23) and Federico Chiesa (21) and goalkeeper Alban Lafont (20).
Fiorentina routinely finish in the top half of Serie A although they have not won a trophy since the Coppa Italia in 2001 and the last of their two league titles was in 1969.
They showed all their potential in thrashing AS Roma 7-1 in a Coppa Italia quarter-final in January but since then, the team's form has dropped.
(Writing by Brian Homewood, editing by Mike Harrison and Christian Radnedge)
