Born in Udine, he studied in his native city and later in Milan with Alfredo Starno, where he made his debut at the Teatro Nuovo in 1957.
After singing with success in various opera houses in Italy, he made his American debut in Cincinnati, as Basilio in Il barbiere di Siviglia, in 1959.
Other roles included Padre Guardiano in La forza del destino, Phillip II in Don Carlo, Ferrando in Il trovatore, Count Walter in Luisa Miller, Zaccaria in Nabucco, Giorgio in I puritani, Commendatore in Don Giovanni, Alvise in La Gioconda, King Heinrich in Lohengrin, He was heard in Donizetti's La Favorita at the Met in 2015 at the age of 81 etc.
While best known for performing the Italian repertoire, Giaiotti did sing a number of non-Italian roles, notably the High Priest in Karl Goldmark's Die Königin von Saba (in 1991 at the Teatro Regio (Turin)), Cléomer in Massenet's Esclarmonde (January 1993, at the Teatro Massimo), Cardinal Brogni in Halevy's La Juive, and the Anabaptist in Meyerbeer's Le Prophète, and, as above, King Heinrich in Richard Wagner's Lohengrin (in 1976 and 1980, at the Metropolitan Opera).
His evenly produced, beautiful and sonorous voice made him one of the leading bassi cantanti of his generation.