Richard Cassilly (December 14, 1927 – January 30, 1998) was an American operatic tenor who had a major international opera career between 1954–90. Cassilly "was a mainstay in the heldentenor repertory in opera houses around the world for 30 years", and particularly excelled in Wagnerian roles like Tristan, Siegmund and Tannhäuser, and in dramatic parts that required both stamina and vocal weight, such as Giuseppe Verdi's Otello and Camille Saint-Saëns's Samson.

He was an admired Don José in Carmen and sang almost all of the leading Puccini tenor roles. Standing at 6'3" and possessing a 250-pound frame The New York Times described him as "a burly tenor with a bright ping on the top notes who had a supple lyric quality [to his voice]", and "was known to bring a musical intelligence and uncommonly clear diction to his work."

Cassilly spent the early years of his opera career singing primarily with the New York City Opera between 1955–1966, often portraying roles in obscure and contemporary operas. During these years he also traveled frequently throughout North America, appearing with most of the major opera companies in the United States and Canada. In 1965 he launched a major international opera career when he portrayed the title role in a critically acclaimed production of Heinrich Sutermeister's Raskolnikoff at the Grand Théâtre de Genève. This performance earned him a contract with the Hamburg State Opera as their leading in-house dramatic tenor, a position he held from 1965 to 1978. Engagements with other major companies soon followed, and by 1973 Cassilly had sung leading roles with almost every major opera house in Europe, including La Scala, the Opéra National de Paris, the Vienna State Opera, and the Bavarian State Opera.

Cassilly also forged a strong collaborative partnership with the Royal Opera in London, appearing in that house almost every year from 1968 to 1982. In 1978 he joined the roster of principal tenors at the Metropolitan Opera in New York City, where he spent the majority of his time until his retirement in 1990.