Sio may refer to:
SIO is a serial port driver package for OS/2 written by Raymond L. Gwinn. It is designed to not only improve performance over OS/2's default serial drivers, but also improve compatibility. SIO contains a virtualized FOSSIL (VX00) driver that can be loaded to provide FOSSIL support to DOS based communications software. SIO later added the ability to create virtualized COM ports, which, combined with the included program VMODEM, allows incoming telnet connections to be directed toward the virtualized COM port. Older communications software are also able to "call out" to telnet sites. This is possible due to a custom set of "AT" commands that allow users to pass a hostname to the VMODEM software.
Silicon monoxide is the chemical compound with the formula SiO where silicon is present in the oxidation state +II. 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.
''Sio nordenskjoldii'', Nordenskjold's bigscale, is a species of ridgehead found in the southern oceans at depths of from 200 to 3000 m ft . This species grows to a length of 12.3 cm in SL. This species is the only known member of its genus. This species is of minor importance to commercial fisheries.