Giovanni Battista Sammartini (c. 1700 – 15 January 1775) was an Italian composer, violinist, organist, choirmaster and teacher. He counted Gluck among his students, and was highly regarded by younger composers including Johann Christian Bach. It has also been noted that many stylizations in Joseph Haydn's compositions are similar to those of Sammartini, although Haydn denied any such influence. Sammartini is especially associated with the formation of the concert symphony through both the shift from a brief opera-overture style and the introduction of a new seriousness and use of thematic development that prefigure Haydn and Mozart. Some of his works are described as galant, a style associated with Enlightenment ideals, while "the prevailing impression left by Sammartini's work... [is that] he contributed greatly to the development of a Classical style that achieved its moment of greatest clarity precisely when his long, active life was approaching its end".
He is sometimes confused with his elder brother, Giuseppe, a composer with a similarly prolific output though not equal renown or influence who ended up in the service of the Prince of Wales.