Nico Castel (born Naftali Chaim Castel Kalinhoff; August 1, 1931 – May 31, 2015) was a Portuguese-born comprimario tenor and 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 in New York City, where he also served as staff diction coach for three decades.

Castel was raised in Venezuela by multilingual parents and a German nanny and attended a French school in Caracas. He moved to the US at the age of 16 to sing and study romance languages at Temple University. In the early 1950s, he served in the United States Army as a translator in Germany. In 1958 he made his debut in Verdi's Falstaff. In 1965, he sang the first of many roles with the New York City Opera. He made his Metropolitan Opera debut in 1970, where he continued to perform until 1997. He was its staff diction coach from 1979 to 2009. Castel had over 200 operatic roles in his repertoire, sang in opera houses in North and South America, Europe and Israel, and appears on various opera recordings.

On the concert stage, Castel often included Jewish music in his programs, and he served as a cantor at synagogues; he wrote a Ladino songbook. He also wrote A Singer’s Manual of Spanish Lyric Diction and translated a series of opera librettos, adding phonetic pronunciations that are used by singers and teachers throughout North America and Europe. He taught at The Juilliard School, Mannes School of Music, Boston University and other universities and conservatories around the world. He and his wife founded and operated the New York Opera Studio, which trained young singers, and he annually presented the Nico Castel International Master Singer Competition.