Which composer is known for works that transition into the Classical period?

The composer known for works that transition into the Classical period is Haydn. He played a pivotal role in the development of chamber music and the symphony, effectively bridging the gap between the Baroque era and the Classical period. Haydn's compositions are characterized by clearer textures, structured forms, and the use of balanced phrasing, hallmarks of the Classical style.

His influence on other composers, particularly Mozart and Beethoven, solidified his role as a key figure in the transition. While Bach and Vivaldi were significant in the Baroque period, their styles and approaches do not reflect the Classical characteristics that emerged later. Beethoven, on the other hand, is often associated with the transition from the Classical to the Romantic period rather than being a transitional figure into the Classical period itself. Thus, Haydn is the composer who best exemplifies the shift towards the Classical style.