The Subansiri River is a tributary of the Brahmaputra River in the Indian states of Assam and Arunachal Pradesh, and the Tibet Autonomous Region of China. The Subansiri is 442 kilometres (275 mi) long, with a drainage basin 32,640 square kilometres (12,600 sq mi) large. The Subansiri is the largest tributary of the Brahmaputra.
Its maximum observed discharge was 18,799 cubic metres per second (663,900 cu ft/s), and its minimum 131 m3/s (4,600 cu ft/s). It contributes 7.92% of the Brahmaputra's total flow. The Subansiri River originates in the Himalayas, in China. It flows east and southeast into India, then south to the Assam Valley, where it joins the Brahmaputra River in Lakhimpur district.