SIW or Siw may refer to:
- Substrate-integrated waveguide, a synthetic rectangular electromagnetic waveguide formed in a dielectric substrate
- Self-inflicted wound, the result of harming oneself
- siw, the ISO 639 code for the Siwai language
- SIW, the IATA code for Sibisa Airport
- SIW, the ICAO airline code for Sirio Executive