The Piano Sonata in B minor (German: Klaviersonate h-Moll), S.178, is a musical composition for solo piano by Franz Liszt, published in 1854 with a dedication to Robert Schumann. It is often considered Liszt's greatest composition for solo piano. The piece has been often analyzed, particularly regarding issues of form.

The Sonata was composed in 1852 and 1853. At this point in his life, Liszt's career as a traveling virtuoso had almost entirely subsided as he had been influenced towards leading the life of a composer rather than a performer by Carolyne zu Sayn-Wittgenstein almost five years earlier. Liszt's life was established in Weimar and he was living a comfortable lifestyle, composing, and occasionally performing, entirely by choice rather than necessity. The sonata was dedicated to Robert Schumann, in return for Schumann's dedication of his Fantasie in C, Op. 17 (1836) to Liszt.

There exists a great deal of speculation surrounding the origins of this piece. While Liszt composed a great deal of programmatic works, at no point did he suggest that this piece was constructed upon any idea greater than pure music. However, it has been suggested that the piece could be programmatic of the Faust legend, be based upon the biblical story of the Garden of Eden or even be biographical. The sonata is constructed as a single movement of non-stop music; however, it is widely believed that Liszt's piece still fits the mold of a traditional four-movement sonata within the mold of one long sonata form.

The quiet ending of the sonata may have been an afterthought; the original manuscript kept in the The Morgan Library & Museum in New York City (also published as a facsimile by G. Henle Verlag in 1973) contains a crossed-out ending section which would have ended the work loudly instead.

The sonata is notable for being constructed from five motivic elements that are woven into an enormous musical architecture. The motivic units undergo thematic transformation throughout the work to suit the musical context of the moment. A theme that in one context sounds menacing and even violent, is then transformed into a beautiful melody. This technique helps to bind the sonata's sprawling structure into a single cohesive unit. Michael Saffle, Alan Walker, and others contend that the first motive appears at the very start of the piece until bar 8, the second occurs from bar 9 until 12 and the third from measures 13 to 17. The fourth and fifth motives appear later in the piece at measures 105-108 and 327-338 respectively.

Broadly speaking, the Sonata has four movements although there is no gap between them. Superimposed upon the four movements is a large sonata form structure, although the precise beginnings and endings of the traditional development and recapitulation sections has long been a topic of debate. Charles Rosen states in his book The Classical Style that the entire piece fits the mold of a sonata form because of the reprise of material from the first movement that had been in D major, the relative major, now reprised in B minor.

Alan Walker, the foremost contemporary Liszt scholar, believes that the development begins roughly with the slow section at measure 331, the leadback towards the recapitulation begins at the scherzo fugue, measure 459, and the recapitulation and coda are at measures 533 and 682 respectively. Each of these sections (exposition, development, leadback, and recapitulation) are examples of Classical forms in and of themselves, which means that this piece is one of the earliest examples of Double-function form, a piece of music which has two classical forms occurring simultaneously, one containing others. For instance the exposition is a sonata form which starts and ends with material in B minor, containing the second part of the exposition and development wandering away from the tonic key, largely through the relative major D. In using this structure, Liszt was influenced by Franz Schubert's Wanderer Fantasy, a work he greatly admired, performed often and arranged for piano and orchestra. Schubert used the same limited number of musical elements to create a broad four movement work, and used a fugal 4th movement. Already in 1851 Liszt experimented with a nonprogrammatic "four-movements-in-one" form in an extended work for piano solo called Grosses Concert-Solo. This piece, which in 1865 was published as a two-piano version under the title Concerto pathétique, shows a thematic relationship to both the Sonata and the later Faust Symphony.

The sonata was published by Breitkopf & Härtel in 1854 and first performed on January 27, 1857 in Berlin by Liszt's pupil and son-in-law, Hans von Bülow