Mario Beretta has returned to Siena as coach in place of Andrea Mandorlini, who was dismissed following Sunday's 3-2 home defeat by Livorno.
Mandorlini, 47, was appointed in June, since when Siena have won just one of their 12 Serie A matches.
They are level with four other teams at the bottom of the table on nine points, but above the drop zone on goal difference.
Mandorlini is replaced by 48-year-old Beretta, who returns to Siena after helping the Tuscan side avoid relegation last term. He previously coached Chievo and Parma in Serie A.
Siena are the third top-flight club to change coach this season following Livorno and Reggina.
Source: C4 Football Italia / UEFA