The Adagio in B minor, K. 540, is a composition for piano solo by Wolfgang Amadeus Mozart. He entered it into his Verzeichnis aller meiner Werke (Catalogue of all my Works) on 19 March 1788.

At 57 measures, the length of the piece is largely based on the performer's interpretation, including the decision of whether to do both repeats; it may last between 5+1⁄2 and 16 minutes. The key of B minor is very rare in Mozart's compositions; it is used in only one other instrumental work, the slow movement from the Flute Quartet No. 1 in D major, K. 285.

In 1841 Franz Xaver Wolfgang Mozart used themes from his father's adagio in his own Fest-Chor.

The Austrian composer and academic Gerhard Präsent has made an arrangement for string quartet in four movements Fantasy Quartet in D in which this piece acts as the third one.^[1]

The autograph^[2] is at the Stiftelsen Musikkulturens Främjande Library, Stockholm.