The club confirmed Cosmi's departure on their official website in a short statement published at midnight.
"AC Siena has dismissed coach Serse Cosmi," the statement said. "The club would like to thank the coach and his staff for the work they have done in recent months with dedication and professionalism."
Sunday's loss came as Fiorentina took a 3-0 lead inside the opening 45 minutes and Siena's misery was completed in the second half by the dismissal of Massimo Paci.
It was a third straight Serie A defeat for the club, leaving them rooted to the foot of the table.
However, their record of four wins, five draws and eight defeats would be good for 15th place were it not for the six-point penalty they were given in the wake of the match-fixing scandal.
Former Sampdoria coach Giuseppe Iachini is now expected to be confirmed as Cosmi's successor.