SIV or Siv may refer to:
- Simian immunodeficiency virus, a virus found in primates and related to HIV
- Siv, a character in the children's fiction book series Guardians of Ga'Hoole
- SIV, the IATA and FAA airport code for Sullivan County Airport, Sullivan County, Indiana, USA
- Simulation d’incidents en vol, or simulation of flying incidents training
- Statens Invandrarverk, the former name of Swedish immigration authority Migrationsverket (on Swedish Wikipedia)
- Stress-induced voiding, a failure mechanism in MOSFETs
- Structured investment vehicle, a type of fund in the shadow banking system
- Swine influenza virus, the cause of influenza in pigs
- Synthetic Initialization Vector, a cryptographic key-wrapping algorithm
- Federal Executive council or Savezno izvršno veće (SIV) in Yugoslavia