The Violin Sonata No. 9 in A major, Op. 47, by Ludwig van Beethoven, is an 1803 sonata for piano and violin notable for its technical difficulty, unusual length, and emotional scope. It is commonly known as the Kreutzer Sonata after the violinist Rodolphe Kreutzer, to whom it was ultimately dedicated, but who thoroughly disliked the piece and refused to play it.

Sonata, in music, literally means a piece played as opposed to a cantata, a piece sung. The term evolved through the history of music, designating a variety of forms until the Classical era, when it took on increasing importance. Sonata is a vague term, with varying meanings depending on the context and time period. By the early 19th century, it came to represent a principle of composing large-scale works. It was applied to most instrumental genres and regarded—alongside the fugue—as one of two fundamental methods of organizing, interpreting and analyzing concert music. Though the musical style of sonatas has changed since the Classical era, most 20th- and 21st-century sonatas still maintain the same structure.