The province of Oristano (Italian: provincia di Oristano; Sardinian: provìntzia de Aristanis) is a province in the autonomous island region of Sardinia, Italy.

It has an area of 3,040 square kilometres (1,170 sq mi), a total population of 160,746 (2016), and a population density of 53.7 people per square kilometer.It is bordered with on the north by province of Sassari, on east by the province of Nuoro, on south by the province of South Sardinia and it is bathed from the Sea of Sardinia to the west.

It occupies roughly the same area as the Giudicato of Arborea of the High Middle Ages.

The province contains Santa Giusta (commune) and Tharros (former city), which both date from the Carthaginian Republic's rule of the area.

The town of Bosa is located in the region alongside a river and its medieval fortifications remain.