His younger brother and successor as president Raul Castro announced the news on state television.
Castro toppled the government in 1959, introducing a Communist revolution. He defied the US for decades, surviving many assassination plots.
His supporters said he had given Cuba back to the people. Critics saw him as a dictator.
Ashen and grave, President Castro told the nation in an unexpected late night broadcast on state television that Fidel Castro had died and would be cremated later on Saturday.