Giovanni Battista Castello was an Italian historical painter.

Born in Gandino near Bergamo, he is ordinarily termed Il Bergamasco to distinguish him from the other painter (of miniatures) with the identical name from school of Genoa. His best-known works are the paintings on the vault of the Basilica della Santissima Annunziata del Vastato. He was an architect and sculptor as well as painter.

When young, he apprenticed with Aurelio Buso of Crema, a pupil of Polidoro da Caravaggio. He was sponsored in Genoa by Tobia Pallavicino and sent to Rome for some years. He returned to decorate the palazzo Pallavicino and the church of San Marcellino. He painted of the Martyrdom of St. Sebastian in the monastery of San Sebastiano. Along with Luca Cambiaso, was commissioned by the Duke Grimaldi to decorate the ceiling of the choir of the Nunziata di Portoria in Genoa, with a fresco of Christ as judge of the world. He worked on various projects with his friend Cambiaso, including in a chapel for the Duomo di San Lorenzo.

He decorated rooms of the Villa Lanzi, Gorlago, near Bergamo, with scenes from the Iliad.

In 1567 he was invited to Madrid to become painter and architect to Philip II. He also executed some works in the Escorial, and died holding the office of architect of the royal palaces, including the Pardo Palace. As architect, he is supposed to have remodeled the church of San Matteo in Genoa and to have designed the imperial palace at Campetto. In 1568, he did the first sketch for the decoration of the Real, the biggest galley of its time. He died at Madrid in 1569 or 1579.