The Tirso (; Latin: Thyrsus) is a river in Sardinia, Italy. It is the longest river on the island and has Sardinia's largest drainage basin. It rises on the plateau of Buddusò, on the slopes of Punta Pianedda, at an elevation of , and flows across the island from east to west, passing through Lake Omodeo before entering the Gulf of Oristano. Its principal tributaries are the Taloro and Flumineddu.
In its upper course, between its sources and the confluence with the Rio Liscoi, the Tirso follows a steep and winding channel. Between the Liscoi and Lake Omodeo, its gradient becomes progressively gentler and its course more regular. In its lower reach across the plain of Oristano, the river has a very low gradient and forms broad meanders.
The basin contains numerous artificial reservoirs, including Lake Omodeo, Lake Gusana, and Lake Cucchinadorza. Lake Omodeo, enlarged by the construction of the , has a maximum storage capacity of about .
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