Pilcomayo (in hispanicized spelling) (Quechua Pillkumayu or Pillku Mayu, pillku red, mayu river, "red river", Guarani Ysyry Araguay [ɨsɨˈɾɨ aɾaˈɣʷaj]) is a river in central South America. At 1,100 kilometres (680 mi) long, it is the longest western tributary of the Paraguay River.