Sio may refer to:
Sio nordenskjoldii, Nordenskjold's bigscale, is a species of ridgehead found in the southern oceans at depths of from 200 to 3,000 metres (660 to 9,840 ft). This species grows to a length of 12.3 centimetres (4.8 in) SL. This species is the only known member of its genus. This species is of minor importance to commercial fisheries.
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.