Giovanni Battista Costanzi was an Italian composer and cellist. He was known with the nickname Giovannino del Violoncello or Giovannino da Roma.
Giovanni Battista Costanzi was probably a pupil of Giovanni Antonio Haym (1660-1729). Costanzi was probably a pupil of Giovanni Lorenzo Lulier. In 1721 he entered the service of Cardinal Pietro Ottoboni, at first as assistant to the room, and then, from 1737 as the succesor of Arcangelo Corelli, as head of the instruments. In 1727 he made his debut as an opera composer with L'amor generoso at the Teatro Capranica in Rome, but his first major success came with his opera Carlo Magno in 1729, after which he was appointed again thanks to the recommendations of Cardinal Ottoboni, chapelmaster in the most important Roman churches.