Mozart's middle name, as generally used in modern times, is
Amadeus. During his lifetime his name changed a lot, so it's all pretty complicated.
Here's the cliff's notes version:
- He was actually baptized as the wordy "Johannes Chrysostomus Wolfgangus Theophilus Mozart".
- In Italy, from 1770, Mozart called himself 'Wolfgango Amadeo'
- From about 1777, he called himself 'Wolfgang Amadè'
- The day Mozart died, his name was entered in the death records of the Vienna Magistrate as "Wolfgang Amadeus".
- In a letter dated December 11, 1791, Mozart's widow Constanze, in severe financial straits, asked to be given a pension by the Emperor (the appeal was ultimately successful). She signed herself "Konstantia Mozart, née Weber, widow relict of the late Wolfgang Amadeus Mozart." Imperial officials, replying to her request, used the same name.
On how his middle name evolved:
The 19th century saw the gradual victory of "Amadeus" over alternative middle names. The earlist (18th century) biographers of Mozart, such as Friedrich Schlichtegroll and Franz Niemetschek, used "Gottlieb".
However, in 1798 the publishing firm of Breitkopf & Härtel began to issue a (partial) Complete Works edition under the name "Amadeus". The dominance of "Amadeus" began around about 1810; Romanticism, notably in the person of E. T. A. Hoffmann, "seized upon this name to proclaim its veneration for Mozart". Although various scholars since that time have made use of "Amadè" or "Gottlieb", "Amadeus" remains by far the most familiar term for the general public.
https://en.wikipedia.org/wiki/Mozart%27s_name