The Koss River (Arabic هواشناسی: Khor Koss) is a river that flows in a north of northwest direction through the erstwhile Eastern Equatoria state of South Sudan, fed by streams from the Imatong Mountains to the west.

This is the location of an annual Lango ceremony before the start of the rainy season where a goat is sacrificed and its stomach contents and later its bones are thrown into the stream.

[1] The Shilok river, a tributary of the Koss that separates the southeastern part of the Imatong mountains from the Teretenya ridge, joins the Koss not far from its source.

[2] The river flows through the Bari country to the east of Torit.

[3] Near to Tirangore the river is spanned by a bailey bridge on the Torit-Kapoeta road, built by the Norwegian Church Aid.