Giustino ("Justin", HWV 37) is an opera seria in three acts by George Frideric Handel. The opera was first given at the Covent Garden Theatre in London on 16 February 1737.The Italian-language libretto was adapted from Charles VI's court poet Pietro Pariati's libretto for Giustino (1711), after the much older original libretto of Nicolò Beregan (1682). The libretto had already been adapted by many composers including Vivaldi's Giustino of 1724 and Tomaso Albinoni's lost opera of 1711.
By the 1736 - 37 season in London, the German-born Handel was presenting both operas he had composed in Italian, as he had done for more than twenty years, and oratorio in English, which was a newer form for him. Giustino was one of three new operas composed by Handel that season. In addition he revived earlier operas and oratorios and presented two new oratorios.
In the middle of all this work, Handel suffered an illness which temporarily left his right hand paralyzed, as reported in the London Evening Post on 14 May 1737:
Handel led the performances of his operas and oratorios from the keyboard and often played organ concertos between the acts; nevertheless he was absent from the theatre while he recovered, which he did fairly speedily although he suffered occasional relapses of this ailment for the rest of his life.
Of the three new operas Handel presented that season, Giustino was the most successful with audiences.
In Constantinople, widowed Empress Arianna is celebrating her marriage to her new husband Anastasio when they receive news that they are threatened by a rebellion. Vitaliano has raised an army and is demanding that Arianna marry him instead and place him on the throne by her side. Anastasio sends a defiant message and sets off to fight the rebel. Arianna determines to follow her husband.
In a rural setting, the ploughboy Giustino is working his fields when he falls asleep and has a vision of great fortune awaiting him. He wakes to see a maiden being chased by a wild bear and rescues her. The maiden turns out to be Princess Leocasta, sister of Anastasio, and, much smitten by her rescuer Giustino, she invites him to the palace where he is acclaimed as a hero and sent to help Empress Arianna and her new husband against the rebel Vitaliano.