Lake Orta (Italian: Lago d'Orta; Lombard and Piedmontese: Lagh d'Òrta) or Cusio (Lombard and Piedmontese: Cusi; Latin: Lacus Cusius) is a lake in northern Italy, west of Lake Maggiore .

It has been so named since the 16th century, but was previously called Lago di San Giulio, after Saint Julius (4th century), the patron saint of the region. Its southern end is about 35 kilometers (22 miles) by rail to the northwest of the city of Novara (located on the main Turin-Milan line), while its northern end is about 6 kilometers (4 miles) by rail south of the Gravellona-Toce railway station, itself located halfway between Ornavasso and Omegna .

Its scenery is characteristically Italian, while San Giulio Island has some picturesque buildings, and takes its name from the local saint, who lived in the 4th century. The island was fortified between the 5th and 6th centuries.

Located around the lake are Orta San Giulio , built on a peninsula projecting from the east shore of the lake, Omegna at its northern extremity, Pettenasco to the east, and Pella to the west.