The Variations for piano in C major on a waltz by Diabelli, most commonly referred to as the Diabelli Variations, are a set of variations for the piano by Ludwig van Beethoven on a waltz by Anton Diabelli. They were written from 1819 to 1823.

The piece came about after Diabelli, a well known music publisher as well as a composer, sent his waltz to a number of composers, asking each of them to write a variation on it. His intention was to publish all the variations in one volume, making a sort of anthology of what he saw as the most important composers of the time. Fifty composers responded, and their variations were published as Vaterländische Künstlerverein (Patriotic Artists' Association). Beethoven, however, rather than writing just one variation, was so taken with the possibilities contained in the waltz that he wrote a set of thirty-three. Diabelli published Beethoven's variations in a separate volume to the rest. They are now seen as one of Beethoven's greatest piano pieces, as well as one of the finest sets of variations of their time.