Gaetano Lo Presti, 52, was found dead in his cell in the Pagliarelli prison in Palermo late on Tuesday night after a massive operation involving 1,200 police officers, the AGR news agency said.
An investigation into how Lo Presti was able to commit suicide has been launched.
Lo Presti, a mafia 'godfather' in Palermo's Porta Nuova district, was among suspects aiming to resurrect the Sicilian mafia, also known as the Cosa Nostra, and has been accused of extortion, arms and drug trafficking.
Mafia members were planning to set up a mafia council and appoint a new godfather, Matteo Messina Denaro, also known as Diabolik. Police believe Lo Presti was one of candidates for the Cosa Nostra leadership.
The Sicilian Mafia, involved in illegal drug and arms trade as well as racketeering, loan sharking and money laundering, was dismantled following the arrest in 2006 of fugitive godfather Bernardo Provenzano, the former "boss of bosses."
