North Sikkim district in the context of Mangan, India

Teesta River (Bengali:তিস্তা,Tistā)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 Jamuna 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 km (4,840 sq mi). In India, the Teesta River flows through Mangan District, Gangtok District, Pakyong District, Kalimpong district, Darjeeling District, Jalpaiguri District, and Cooch Behar District as well as the cities of Rangpo, Jalpaiguri, and Mekhliganj. In Bangladesh, it flows through Lalmonirhat District, Rangpur District, Kurigram District and Gaibandha District. It joins the Jamuna 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.

Gangtok district is an administrative district of the Indian state of Sikkim. It was renamed in 2021 as a result of administrative reorganisation of the state, which also saw three subdivisions of the East Sikkim district spawned off as a separate Pakyong district.

The headquarters of the Gangtok district is Gangtok, which is also the state capital. Gangtok is the hub of all administrative activity in the state. The district is bounded by the Pakyong district in the south and southeast, Bhutan in the east, the People's Republic of China in the northeast, Mangan District in the north and Namchi District in the west.