The Kaso River (Sungai Kaso or Kacu) is a stream in the Indonesian part of the island of Borneo, about 1100 km northeast of the Indonesian capital Jakarta.

[2] In 1889, Tromp reported three villages of Bukat people on the Kaso.

The villages were on the Penane River, a tributary of the Kacu at Long Mecai.

The semi-nomadic Seputan people also live on the Kacu at times.

The warmest month is September when the average temperature is around 22 °C, and the coldest is June, at 20 °C.