The Piano Sonata No. 14 in C♯ minor "Quasi una fantasia", Op. 27, No. 2, popularly known as the Moonlight Sonata, is a piano sonata by Ludwig van Beethoven. It was completed in 1801 and dedicated in 1802 to his pupil, Countess Giulietta Guicciardi.

This piece is one of Beethoven's most popular compositions for the piano, and it was a popular favorite even in his own day.Beethoven wrote the Moonlight Sonata in his early thirties, after he had finished with some commissioned work; there is no evidence that he was commissioned to write this sonata.

The first edition of the score is headed Sonata quasi una fantasia, a title this work shares with its companion piece, Op. 27, No. 1. Grove Music Online translates the Italian title as "sonata in the manner of a fantasy". Translated more literally, this is "sonata almost a fantasy".

The name "Moonlight Sonata" comes from remarks made by the German music critic and poet Ludwig Rellstab. In 1832, five years after Beethoven's death, Rellstab likened the effect of the first movement to that of moonlight shining upon Lake Lucerne. Within ten years, the name "Moonlight Sonata" ("Mondscheinsonate" in German) was being used in German and English publications. Later in the nineteenth century, the sonata was universally known by that name.

Many critics have objected to the subjective, romantic nature of the title "Moonlight", which has at times been called "a misleading approach to a movement with almost the character of a funeral march" and "absurd". Other critics have approved of the sobriquet, finding it evocative or in line with their own interpretation of the work. Gramophone founder Compton Mackenzie found the title "harmless", remarking that "it is silly for austere critics to work themselves up into a state of almost hysterical rage with poor Rellstab", and adding, "what these austere critics fail to grasp is that unless the general public had responded to the suggestion of moonlight in this music Rellstab's remark would long ago have been forgotten."

Although no direct testimony exists as to the specific reasons why Beethoven decided to title both the Op. 27 works as Sonata quasi una fantasia, it may be significant that the layout of the present work does not follow the traditional movement arrangement in the Classical period of fast–slow–[fast]–fast. Instead, the sonata possesses an end-weighted trajectory, with the rapid music held off until the third movement. In his analysis, German critic Paul Bekker states: "The opening sonata-allegro movement gave the work a definite character from the beginning... which succeeding movements could supplement but not change. Beethoven rebelled against this determinative quality in the first movement. He wanted a prelude, an introduction, not a proposition.”

The first movement, in C♯ minor, is written in an approximate truncated sonata form, especially considering the "introduction" function of mm. 1–5. The movement opens with an octave in the left hand and a tr