Dubbed the “people slayer”, Brusca has confessed to his role in over 100 killings, including the assassination of Italy’s top anti-Mafia prosecutor Giovanni Falcone.
But Brusca became an informant, helping prosecutors hunt down fellow mobsters.
His release after 25 years in jail has outraged his victims’ relatives.
He will now be on parole for four years.
Who is Giovanni Brusca?
Brusca, now 64, was a key figure within the Cosa Nostra, the Sicilian Mafia group.
Mr Falcone’s wife and three bodyguards were also killed in the attack, when Brusca set off half a tonne of explosives under the road near Palermo they were driving along.
The attack, followed two months later by the killing of Mr Falcone’s colleague Paolo Borsellino, rocked Italy and resulted in tough new anti-Mafia laws.
Brusca has confessed to his role in more than 100 murders.
After his arrest in 1996, he turned state witness in order to reduce his sentence. He helped investigators track down the gangsters responsible for several Mafia attacks in the 1980s and 1990s.
What has the reaction been?
Brusca’s release has prompted grief and anger among relatives of some of his victims.
The wife of one of the bodyguards killed, Tina Montinaro, told the Repubblica newspaper she was “indignant”.
“The state is against us – after 29 years we still don’t know the truth about the massacre and Giovanni Brusca, the man who destroyed my family, is free,” Ms Montinaro said.
Maria Falcone, the sister of the judge, said she was “saddened” by the news but that the law gave Brusca the right to leave prison.
Several Italian politicians condemned Brusca’s release.
“After 25 years in prison, the mafia boss Giovanni Brusca is a free man. This is not the ‘justice’ that Italians deserve,” said Matteo Salvini, leader of the right-wing League party.