The Allegro in C, K. 1b, is a keyboard work written by Wolfgang Amadeus Mozart in 1761. It is part of a group of keyboard works found in Nannerl's Music Book. It is an extremely short piece (just 15 seconds long), likely notated by his father, Leopold Mozart, since Wolfgang was six years old at the time.

It is normally performed on the harpsichord, and is in the key of C. As the word allegro in its name suggests, it is a fast and lively piece. The piece is unlike Mozart's only previous work - the Andante in C, in that it does not operate on repeated phrases. It begins with climbing fifths, which, after reaching a peak, drop in triplets before concluding in a triplet chord.