Fordongianus (; Ancient Greek: Hydata Hypsitana; or Forum Trajani) is a comune (municipality) in the Province of Oristano in the Italian region of Sardinia, located about northwest of Cagliari and about northeast of Oristano. As of 31 December 2004, it had a population of 1,037 and an area of .
Fordongianus borders the following municipalities: Allai, Busachi, Ghilarza, Ollastra, Paulilatino, Siapiccia, Villanova Truschedu.
In antiquity, Fordongianus was called Forum Trajani in honor of Roman emperor Trajan, who is credited with the building of what are now considerable Roman remains, including those of a bridge, and of thermae on a scale of great magnificence (Valéry, Voy. en Sardaigne, vol. ii. c. 35). The city, in the interior of Sardinia, is known from the Itineraries, which place it on the road from Tibula, through the interior of the island, to Othoca. (Itin. Ant. p. 82.) Fordongianus sits on the left bank of the river Tirsi (ancient Thyrsus), about from Oristano.