Lhonak Valley is located in the northwestern corner of Sikkim state of India. It is trans-Himalayan grassland in the exposed river valley of Goma Chu, with boggy marshes, glacial lakes, barren screed slopes and glaciers. Goma Chu rises in North and South Lhonak glaciers and runs across the valley to join Zema Chu, a glacier at the southern end of the valley, as is the Green Lake.

It is accessible from Thangu via Lungnak La pass which is 5,900 meters high. This Valley is the only known breeding area in the Eastern Himalayas of the Black-necked Crane Grus nigricollis. Lakes and marshes here are used as stopover sites for migratory water birds and support populations of the Sikkim Snow Toad Scutiger.

Vegetation is typical cold desert, with xerophytic species such as Ephedra gerardiana, herbs, grasses and sedges, aquatic weeds and many medicinal and commercially valuable plants such as Picrorhiza kurrooa and Meconopsis horridula.

The inhabitants of the valley are herdsmen, the graze yaks, sheep and goats. The Lhonak valley remains cut off from the rest of the country for more than eight months of the year. Due to heavy snowfall it remains inaccessible in winters and in monsoon it becomes difficult to cross Lungnak La pass. High alpine valley of Lhonak is only inhabitable for rare high altitude ruminants and their predators.