Johann Kuhnau (German: [ˈkuːnaʊ]; 6 April 1660 – 5 June 1722) was a German polymath, known primarily as a composer today. He was also active as a novelist, translator, lawyer, and music theorist, and was able to combine these activities with his duties in his official post as Thomaskantor in Leipzig, which he occupied for 21 years. Much of his music, including operas, masses, and other large-scale vocal works, is lost. His reputation today rests on his Biblical Sonatas, a set of programmatic keyboard sonatas published in 1700, in which each sonata depicted in detail a particular story from the Bible. After his death, Kuhnau was succeeded as Thomaskantor by Johann Sebastian Bach.

Much of the biographical information on Kuhnau is known from an autobiography published by Johann Mattheson in 1740 in his Grundlage einer Ehrenpforte. Kuhnau's Protestant family were originally from Bohemia, and their name was Kuhn. Kuhnau was born in Geising, present-day Saxony. His musical talents were apparent early, and in around 1670, he was sent to Dresden to study with court musicians there. During the next decade, he studied keyboard playing and music composition, as well as French and Italian. In 1680, an offshoot of the Great Plague of Vienna reached Dresden, and Kuhnau returned home. He subsequently studied music at the Johanneum in Zittau, and then law at Leipzig University. Exceptionally active as a composer and performer during his university years, he was appointed organist of Leipzig's Thomaskirche in 1684, at the age of 24.

In 1688, Kuhnau completed his dissertation and began practising law. He was still working as an organist and continued composing. In 1689 he published his first collection of keyboard works, followed by three more in 1692, 1696, and 1700. During the 1690s, he translated a number of books into German from Italian and French, completed and published his best-known novel, the satirical Der musicalische Quack-Salber (1700), and devoted his spare time to studying various subjects such as mathematics, Hebrew and Greek. In 1701 he succeeded Johann Schelle as Thomaskantor and kept the position until his death. Although he was successful in directing the many musical activities of the Thomaskirche and teaching at Thomasschule, Kuhnau started suffering from bad health. Scholar Willi Apel noted that the job was "as vexatious and difficult for him as for his successor, J.S. Bach." Not only health troubles, but also efforts by rival musicians and composers such as Georg Philipp Telemann and Kuhnau's own student Johann Friedrich Fasch, undermined his activities as Kantor.

Kuhnau died in Leipzig on 5 June 1722. He was survived by three daughters, from a marriage of 1689. His pupils included not only Fasch, but also Johann David Heinichen and Christoph Graupner.

Kuhnau's reputation today rests on four collections of music for keyboard, which he published in 1689–1700. Particularly important is the last volume, titled Musicalische Vorstellung einiger biblischer Historien, and known popularly as the Biblical Sonatas. It contains six sonatas, each outlining a biblical story in several contrasting movements:

Kuhnau uses a wide variety of musical devices to portray the series of events (the sounding of trumpets, the hurling of David's stone, etc.) as well as the characters' psychological states (e.g. the Israelites' fright before a battle, or Hezekiah's joy darkened by a remembrance of his illness). These devices are not limited to changes of texture or harmony, but also include quotations from Protestant chorales (the Israelites' prayer is based on Luther's Aus tiefer Not schrei ich zu dir) and imitations of operatic arias (Gideon's fear).

The other keyboard works by Kuhnau show a varied approach to form. The two parts of his Clavier-Übung include 7 suites each. The first is only in the major mode, and the second is only in the minor mode. The suites almost always begin with a prelude, and continue through the usual order of dances – allemande, courante, sarabande, gigue – occasionally with a minuet or aria placed between the dances. Kuhnau's preludes are almost always in two sections: a prelude and a fugue (or a fugato section), complete with countersubjects Kuhnau mentions in the preface. Kuhnau's Sonata in B-flat major, appended to the Neuer Clavier-Übung, anderer Theil, was for some time considered to be the earliest known keyboard sonata. Later research has shown that it was rather the first keyboard sonata published in Germany and that Kuhnau simply followed the naming convention established by contemporary foreign composers. The composer himself commented on the issue in the preface:

I have also appended a Sonata in B-flat major, which should also be pleasing to the amateurs. Why shouldn't one provide such pieces for keyboard which are provided for other instruments? Indeed, no instrument has been able to dispute the clavier's reputation for perfection.