Completed near the end of his life, Prokofiev's Ninth Piano Sonata was his least successful, and remains his least known, work in this genre. It was also his last complete piano composition. Unlike the preceding Sonatas Nos. 6-8—the so-called "War sonatas," which are epic, emotionally intense works—the Ninth Sonata is a return to the intimacy of the Fourth Sonata of 1917. In general, the gentle lyricism and simplicity of Prokofiev's earlier works are in evidence in this piece, and familiar techniques, such as diatonicism, simple rhythms, and witty harmonic/cadential surprises are in abundance. The work is dedicated to pianist Sviatoslav Richter.

The Ninth Sonata has four movements: Allegretto, Allegro strepitoso, Andante tranquillo, and Allegro con brio, ma non troppo presto. Musicologist Israel Nestyev has remarked on the similarities between thematic material in the Ninth Sonata and some of Prokofiev's ballet music, including Cinderella, and especially War and Peace.

The opening movement, typically, is in sonata form and, in Nestyev's words, "is miniature in form, modestly developed, and devoid of dramatic conflicts." The second movement is a scherzo in which virtuosic figures appear. The third movement, like the first, is lyrical and melodic, with simple, song-like melodies. The finale is energetic and lighthearted, cast in sonata-rondo form, with more virtuosic running passages and challenging figurations; as in the Fourth Sonata, the finale includes material similar in character to the Commedia del arte-inspired clownishness of the Second Sonata. By the end of the finale, however, the gentle lyrical theme from the first movement returns, and the work concludes peacefully.

This last sonata exemplifies a number of stylistic traits of "late Prokofiev," most significantly the sophisticated harmonic language; however, these complex harmonies are juxtaposed with a disarmingly simple lyricism, resulting in what pianist Murray McLachlan has called a sonata of "contrasts and contradictions," a work "deeply felt and unified." The novelty with which Prokofiev achieves unity in this sonata is perhaps the work's most important feature: at the end of each of the first three movements, the opening theme of the following movement is heard. This thematic foreshadowing links the sonata's movements together, and this, along with the usual thematic/motivic integration, strongly unifies the work. Perhaps, after the bombast and intensity of the War Sonatas, simplicity and gentleness of the Ninth Sonata seemed somehow lacking, or a regression, and despite its many qualities, the work was not published until eight years after it was completed, and is rarely performed.

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