SIW (disambiguation)
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

People 
 Siw Malmkvist, a Swedish schlager singer and actress
 Siw Hughes, a Welsh actress
 Siw Anita Andersen, a Norwegian actress and comedian
 Siw Wittgren-Ahl, a Swedish social democratic politician

See also 
 Siv (given name)