Nico Castel (August 1, 1931 – May 31, 2015), born Naftali Chaim Castel Kalinhoff, was a comprimario tenor and well-known language and diction coach, as well as a prolific translator of libretti and writer of books on singing diction. Although Castel performed throughout Europe, North America and South America, he was best known for his nearly 800 performances at The Metropolitan Opera, where he also served as staff diction coach for three decades.