Teesta River is a 414 km (257 mi) long river that rises in the Pauhunri Mountain of eastern Himalayas, flows through the Indian states of Sikkim and West Bengal and subsequently enters Bangladesh through Rangpur division. In Bangladesh, it merges with Brahmaputra River which after meeting some other major rivers of the Bengal delta finally falls into the Bay of Bengal. It drains an area of 12,540 km2 (4,840 sq mi). In India, the Teesta flows through Mangan District, Gangtok District, Pakyong District, Kalimpong district, Darjeeling District, Jalpaiguri District, Cooch Behar districts and the cities of Rangpo, Jalpaiguri and Mekhliganj. In Bangladesh, it flows through Lalmonirhat District, Rangpur District, Kurigram District and Gaibandha District. It joins the Brahmaputra River at Phulchhari Upazila in Bangladesh. 305 km (190 mi) of the river lies in India and 109 km (68 mi) in Bangladesh. The Teesta is the largest river of Sikkim and second largest river of West Bengal after the Ganges.

Sikkim is a state in northeastern India. It borders the Tibet Autonomous Region of China in the north and northeast, Bhutan in the east, Koshi Province of Nepal in the west, and West Bengal in the south. Sikkim is also close to the Siliguri Corridor, which borders Bangladesh. Sikkim is the least populous and second-smallest among the Indian states. Situated in the Eastern Himalaya, Sikkim is notable for its biodiversity, including alpine and subtropical climates, as well as being a host to Kangchenjunga, the highest peak in India and third-highest on Earth. Sikkim's capital and largest city is Gangtok. Almost 35% of the state is covered by Khangchendzonga National Park – a UNESCO World Heritage Site.