Delio Rossi quit as Lazio coach after deciding he did not have the full backing of the Rome club.
Lazio lifted the Italian Cup last month but finished 10th in Serie A.
“It has not been an easy decision,” Rossi told a news conference. “I felt I was no longer at the centre of the Lazio project.”
Club president Claudio Lotito had refused to confirm Rossi in the job in recent interviews while media reports have said former Palermo boss Davide Ballardini has been lined up for the role.
Rossi, 48, took over in 2005 and guided them to a third-placed finish in 2007 and Champions League football the following season.
Quizzed about speculation he would be appointed at relegated Torino, Rossi said: “Now I don’t know what I will do, I don’t have another team. The division doesn’t matter.”