A nocturne (from the French for "nocturnal") is usually a musical composition that is inspired by, or evocative of, the night.

The name nocturne was first applied to pieces in the 18th century, when it indicated an ensemble piece in several movements. Sometimes it carried the Italian language equivalent of nocturne, notturno, Wolfgang Amadeus Mozart's Serenata Notturna being one example. At this time, the piece was not necessarily evocative of the night, but might merely be intended for performance at night, much like a serenade.

In its more familiar form as a single-movement character piece usually written for solo piano, the nocturne was cultivated primarily in the 19th century. The first nocturnes to be written as such were by the Irish composer John Field, generally viewed as the father of the modern nocturne. However, the most famous exponent of the form was Frederic Chopin, who wrote 21 of them. Later composers to write nocturnes for the piano include Gabriel Fauré and Erik Satie. One of the most famous pieces of 19th century salon music was the "5th Nocturne" by Ignace Leybach , who is now otherwise forgotten.

Other examples of nocturnes include the one for orchestra from Felix Mendelssohn's incidental music for A Midsummer Night's Dream and the set of three for orchestra and female choir by Claude Debussy (who also wrote one for solo piano).

The first movement of Ludwig van Beethoven's Moonlight Sonata has also been considered a nocturne (certainly, Ludwig Rellstab, who gave the piece its nickname, thought it evocative of the night), although Beethoven did not describe it as one.

Nocturnes are generally thought of as being tranquil, often expressive and lyrical, and sometimes rather gloomy, but in practice pieces with the name nocturne have conveyed a variety of moods: the second of Debussy's orchestral Nocturnes, "Fêtes", for example, is very lively.

The word was later used by James McNeill Whistler in the title of a number of his paintings. Several other artists followed suit.

"Nocturne" in popular culture

Like most terms of classical music, "nocturne" has found applications in popular culture. For example, "Nocturne" is the title of an album by Siouxsie and the Banshees. "Nocturne " is also the title of Secret Garden's winning 1995 Eurovision Song Contest entry, and it is also the name of a horror PC game.