The Ksi Sii Aks River (or Tseax') is a tributary of the Nass River in northwestern British Columbia, Canada. It is most notable as the namesake of Tseax Cone, a volcano within its basin that was responsible for an eruption that killed 2,000 Nisga'a people. Prior to the eruption, the Nisga'a name for this river was Ksi Gimwits'ax. Buried by the eruption, it eventually resurfaced.