The Piano Sonata in B-flat major, K. 498a (Anh. 136), is a piano sonata in four movements. It was first printed in 1798 by P. J. Thonus in Leipzig on behalf of Breitkopf & Härtel and attributed to Wolfgang Amadeus Mozart; an edition printed in c. 1805 already credited it as opus 26 of the Thomascantor August Eberhard Müller (1767–1817). Some publications still attribute it to Mozart, often as Piano Sonata No. 20.

Date of composition 1798
Type Sonata
Tonality B-flat Major
Catalogue KV 498a
Instruments Piano
Autotranslations beta Wolfgang Amadeus Mozart: Sonate pour piano en si bémol majeur, KV 498a
Wolfgang Amadeus Mozart: Sonata per pianoforte in si bemolle maggiore, KV 498a
Wolfgang Amadeus Mozart: Sonate für Klavier B-dur, KV 498a