Nico Castel (born 1 August 1931, Lisbon, Portugal - died 31 May 2015, New York City, New York, USA) was a Portuguese-born American operatic comprimario tenor, language and diction coach, translator of libretti and writer of books on singing diction. Castel performed throughout Europe, North America and South America and gave nearly 800 performances at The Metropolitan Opera, where he also served as staff diction coach for three decades.