Sio may refer to:

• Sió, an artificial channel in Hungary
• Sio, a commune in Mali
• Sui, a town in Papua New Guinea
• The IATA airport code for Smithton Airport in Tasmania, Australia

Sió is an artificial channel in central Hungary. It is the outlet of Lake Balaton. It flows out of the lake in the city Siófok. It flows along the city Szekszárd, and a few km east of this city it flows into the river Danube. The river Sió flows through the Hungarian counties Somogy, Fejér and Tolna.

Siø is a small Danish island in the South Funen Archipelago between Tåsinge and Langeland. It is part of Langeland Municipality. With an area of 1.3 km², as of 1 January 2014 it has a population of 16. Since 1681, a system of dykes has expanded it to include the former islands of Skovø and Store Fugleø. In 1959, it was connected to Tåsinge by a causeway and a low bridge and, in 1962, to Langeland by a bridge. Today the island is used mainly for pig farming.

Silicon monoxide is the chemical compound with the formula SiO where silicon is present in the oxidation state +2. In the vapour phase it is a diatomic molecule. It has been detected in stellar objects and it has been described as the most common oxide of silicon in the universe.
When SiO gas is cooled rapidly, it condenses to form a brown/black polymeric glassy material, (SiO)_n, which is available commercially and used to deposit films of SiO. Glassy (SiO)_n is air- and moisture-sensitive. Its surface readily oxidizes in air at room temperature, giving an SiO₂ surface layer that protects the material from further oxidation. However, (SiO)_n irreversibly disproportionates into SiO₂ and Si in a few hours between 400 and 800°C, and very rapidly between 1,000 and 1,440°C, although the reaction does not go to completion.