The Sunkoshi, also called Sunkosi, is a trans-boundary river that originates in Tibet Autonomous Region and is part of the Koshi or Saptkoshi River system in Nepal.

The upper course or main tributary is known as the Bhote Koshi in Nepal and Poiqu River in Tibet.

The Sunkoshi’s headwaters are located in the Zhangzangbo Glacier in Tibet.Both Sunkoshi and Bhotekoshi river courses together form one basin that covers an area of about 3,394 km² (1,310 sq mi).

The Indravati meets the Sunkoshi at Dolaghat, up to where it is followed by the Arniko Rajmarg.From there, the Sunkoshi flows eastwards through the valley formed between the Mahabharat Range and the Himalayas. Tamakosi, Likhu, Dudhkosi, Arun and Tamor are its left tributaries and Indravati is the right tributary.

The average annual flow is 22 x 109 m3. The average sediment load is 54 x 106 m3.

The Koshi River drains eastern Nepal. It is also known as Saptkoshi River because of the seven rivers joining in east-central Nepal to form this river. The main rivers forming the Koshi River system are Sunkoshi, Indravati, Tamba Koshi, Bhote Koshi, Dudh Kosi, Arun and Tamur Rivers. The Saptkoshi River flows through the Chatra Gorge in a southerly direction into northern Bihar and joins the Ganges.

The Sunkoshi contributes about 44% of the total water of the Saptakoshi, the Arun 37% and the Tamur 19%.