The 51-year-old man, whose name wasn't reported, apparently used his shoelaces to hang himself from the window grating of his cell, Italy's ANSA news agency reported.
He had been jailed since 2006 for a series of robberies and his sentence was up in 2014.
Some 118 inmates have died since the beginning of the year, 41 of them suicides.
Penitentiary Police Union leader Leo Beneduci said that the prison situation "is a full-out massacre that needs to be stopped at all costs."
"Aside from the powerlessness in front of daily tragedies, the men and women who work in the penitentiaries are frustrated by the inertia of the administrators and politicians who have abandoned them," Beneduci said.
Notable deaths of 2014 [PHOTOS]