Siat is a former municipality in the district of Surselva in the Swiss canton of Graubünden. On 1 January 2014 the former municipalities of Siat, Castrisch , Ilanz, Ladir , Luven , Pitasch , Riein , Ruschein , Schnaus , Sevgein , Duvin , Pigniu and Rueun merged into the new municipality of Ilanz/Glion .

Siat is first mentioned about 840 as Septe. Until 1943 Siat was known as Seth.

Before the merger, Siat had a total area of 13.5 km2 (5.2 sq mi). Of this area, 51.1% is used for agricultural purposes, while 33.6% is forested.