SIO is a three letter acronym that could mean:

• Scripps Institution of Oceanography
• The IATA airport code for Smithton Airport in Tasmania, Australia
• Serial Input/Output, see Serial cable
• Senior Investigating Officer, see Policing in the United Kingdom
• Students Islamic Organisation of India
• Societas Internationalis Odonatologica (International Dragonfly Society)
• The SIO FOSSIL driver for OS/2, written by Raymond L. Gwinn
• Studentsamskipnaden i Oslo - Foundation for Student Life in Oslo

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