Susannah is an opera in two acts by the American composer Carlisle Floyd, who wrote the libretto and music while a member of the piano faculty at Florida State University. Floyd adapted the story from the Apocryphal tale of Susannah and the Elders, though the latter story has a more positive ending. The story focuses on 18-year-old Susannah Polk, an innocent girl who is targeted as a sinner in the small mountain town of New Hope Valley, in the Southern American state of Tennessee.

The opera was awarded the New York Music Critics Circle Award for Best New Opera in 1956 and was chosen to represent American music and culture at the World's Fair at Brussels in 1958, with a production (by Frank Corsaro) that featured Phyllis Curtin and Norman Treigle. It received its Metropolitan Opera premiere in 1999, with Renée Fleming singing the title role, Jerry Hadley singing Sam and Samuel Ramey singing Blitch. Ramey also recorded the complete opera with Cheryl Studer as Susannah and Jerry Hadley as Sam. Other well-known sopranos who have portrayed the heroine have included Lee Venora, Joy Clements, Faith Esham, Maralin Niska, Nancy Shade, Diana Soviero, Karan Armstrong, Kelly Kaduce and Phyllis Treigle (opposite Michael Devlin as Blitch).

Susannah is one of the most performed American operas, second to Porgy and Bess, and celebrated its 50th anniversary with a performance on the very stage where it premiered February 24, 1955, in Ruby Diamond Auditorium at Florida State University. At the first performance, Carlisle Floyd was awarded an Honorary Doctorate of Humane Letters from Florida State.

It has been speculated that the opera was inspired by McCarthyism, and threat of institutional Hollywood communist invasion of America during the early 1950s. The opera also contains many feminist themes that had not been widely explored in popular culture at the time of the opera's writing. Floyd said this opera was meant to be different from a traditional opera.

The music is largely characterized by Appalachian folk melodies. Also included are some Protestant hymns and some traditional classical music. A particularly prominent part of the opera is Susannah's soaring and melancholy aria in Act II, "The Trees on the Mountain", which is similar to Appalachian folk tunes but in fact Floyd's own composition. In 2019, Rhiannon Giddens released a vernacular version of "Trees on the Mountains" on her album, There Is No Other.