Giovanni Battista Mancini (1 January 1714 – 4 January 1800) was an Italian soprano castrato, voice teacher, and author of books on singing.
Mancini was born at Ascoli Piceno, Italy. He studied singing in Naples with Leonardo Leo and in Bologna with Antonio Bernacchi. He also studied composition and counterpoint there with Giovanni Battista Martini.
He began his singing career when he was 16, appearing in both Italy and Germany. His even greater success as a teacher led to an invitation in 1757 by Empress Maria Theresia of Austria to become "k. k. Cammer-Musicus" and thereby teach her daughters singing. In 1774 in Vienna he published an important book on singing Pensieri, e riflessioni pratiche sopra il canto figurato. His views and publications brought him into conflict with another voice teacher, Vincenzo Manfredini. He remained in Vienna and died there as "Retired Singing Master", leaving behind a considerable fortune.