(b Brescia, c1550; d 19 Jan 1608). Italian instrumental virtuoso. He was praised while still a young man for his mastery of counterpoint, his lute playing and above all for his talent as a viol player. In 1571 Duke Guglielmo Gonzaga offered him a high salary to go to Mantua, but Jacomelli declined; in 1576 after similar negotiations he still preferred to remain in Rome, giving as principal reasons Mantua's bad climate and the plague. He was approached yet again in 1581, but stayed in Rome, where he served briefly as maestro di cappella at the confraternity of S Rocco in 1574–5, a body that he worked for later in his career; he was also employed by the confraternity of the Gonfalone. He served briefly as organist at S Giovanni in Laterano in 1581, and later that year he joined the papal choir as a tenor, a post that he held until 1585 when he was expelled for joining the Congregazione dei Musici di Roma. At this time his harp playing drew much attention, and in 1586 G.M. Nanino called him the best violinist in Rome. In May of the same year he finally entered the service of the Gonzaga court where he was assured of a lifelong position and received a high salary. After a short time, and probably because he had been denied the post of maestro di cappella, Jacomelli gave up the position. Shortly afterwards he entered the service of the Medici in Florence, and in 1589 he participated in the intermedi performed with Bargagli's La pellegrina to mark the marriage of Ferdinando de' Medici and Christine of Lorraine. In Florence he also worked at the Compagnia dell'Arcangelo Raffaello, at SS Annunziata, where he played the organ. In Peri's foreword to Euridice, published in 1601, Jacomelli is said to be ‘most excellent in every part of music, who has almost changed his name to Violino, being a marvellous violinist’; Peri also wrote that in the performance of Euridice in Florence on 6 October 1600 ‘Messer Giovanbattista dal Violino’ played a lira grande. In 1603 he is mentioned, together with Caccini, as one of the best-paid musicians of the Medici court in Florence.
Although he was praised for his command of counterpoint, his compositions were not held in high esteem in Mantua according to Pompeo Strozzi, the chargé d'affaires in Rome. Only one of his works survives, the eight-voice motet Benedicam Domino, published in Carlo Berti's Motecta (Venice, 1596). However, his development as a virtuoso is particularly interesting; he first strove for perfection on the viola da gamba, later turned to the harp, and during the 1580s became prominent as one of the earliest violin virtuosos.