Damiano Beltotto said his grandfather died of cardiac arrest in his sleep.
The singer was born Pietro Ferraro in 1924 in Altivole in northeast Italy. He took the stage name Pier Miranda Ferraro, borrowing the middle name from his wife's name.
Ferraro made his debut at Milan's La Scala opera house in 1951 and last performed there in 1972, family members said.
He sang on the stages of Covent Garden in London and the Vienna State Opera, as well as in operas performed in South America.
His last stage performance was in 1981 in the northern Italian town of Lecco in a production of "Otello."
In 1985, Ferraro founded a management agency for opera singers, conductors, directors and other opera professionals. Ferraro's daughter Beatrice is among those who run the Milan-based agency.
The singer is survived by his wife, four children and 16 grandchildren. A funeral was scheduled for Monday in a Milan church.