Sonata for Piano and Violin No. 21 in E minor (K. 304/300c) is a work by Wolfgang Amadeus Mozart. It was composed in 1778 while Mozart was in Paris. The piece was composed during the same period that Mozart's mother, Anna Maria Mozart, died, and the sonata's mood reflects this. It is the only violin sonata by Mozart in a minor key.
Martin-Joseph Mengal arranged the two movements to wind quintet and incorporated them in his Quintetto tiré des à Âuvres de Mozart.