The Nilgiri Biosphere Reserve is a biosphere reserve in the Nilgiri Mountains of the Western Ghats in South India. It is the largest protected forest area in India, spreading across Tamil Nadu, Karnataka and Kerala. It includes the protected areas Mudumalai National Park, Mukurthi National Park, Sathyamangalam Wildlife Sanctuary in Tamil Nadu; Nagarhole National Park, Bandipur National Park, both in Karnataka; Silent Valley National Park, Aralam Wildlife Sanctuary, Wayanad Wildlife Sanctuary, and Karimpuzha Wildlife Sanctuary in Kerala.

An ecosystem of the hill ranges of Nilgiris and its surrounding environments covering a tract of over 5000 square kilometers was constituted as Nilgiris Biosphere Reserve by UNESCO in September 1986 under Man and Biosphere Programme. Nilgiris Biosphere Reserve is India's first and foremost biosphere reserves with a heritage, rich in flora and fauna. Tribal groups such as the Badagas, Toda, Kotas, Irulla, Kurumba, Paniya, Adiyan, Edanadan Chettis, Allar, and Malayan are native to the reserve.

The word Nilgiri is derived from the Tamil word neelam meaning blue and the Sanskrit word giri meaning mountain. It is thought that the bluish flowers of kurinji shrubs gave rise to the name.

In the 1970s, an area of around 5,670 km (2,190 sq mi) in the Nilgiri Mountains was proposed to be included in the list of biosphere reserves of India. This proposed area encompassed a forestry zone of 2,290 km (880 sq mi), a core zone of 2,020 km (780 sq mi), an agricultural zone of 1,330 km (510 sq mi) and a restoration zone of 30 km (12 sq mi). Nilgiri Biosphere Reserve was established in September 1986 and is India's first biosphere reserve under UNESCO's Man and the Biosphere Programme.

Nilgiri Biosphere Reserve extends over an area of 5,520 km (2,130 sq mi) from the eastern part of Kodagu District to Erode District in the east and to the Palakkad Gap in the south with an elevation of 80 to 2,600 m (260 to 8,530 ft). It has a buffer zone of 4,280 km (1,650 sq mi) and core areas of 1,250.3 km (482.7 sq mi), comprising 701.8 km (271.0 sq mi) in Karnataka, 264.5 km (102.1 sq mi) in Kerala and 274 km (106 sq mi) in Tamil Nadu.

The reserve extends from the tropical and subtropical moist broadleaf forests, tropical moist forests of the western slopes of the Ghats to the tropical and subtropical dry broadleaf forests tropical dry forests on the east slopes. The rainfall range is 500–7,000 mm (20–276 in) per year. The reserve encompasses three ecoregions, the South Western Ghats moist deciduous forests, South Western Ghats montane rain forests, and South Deccan Plateau dry deciduous forests.

Nilgiri Biosphere Reserve harbours more than 3,700 plant species, including about 200 medicinal plants; the 132 endemic flowering plants are contained in the list of endemic plants in the Nilgiri Biosphere Reserve. Stunted evergreen trees grow in shola forest patches above 1,800 m (5,900 ft) and are festooned with epiphytes.

Tall trees above a height of 18 m (59 ft) are used by the giant honey bees (Apis dorsata) for building nests, including the species Tetrameles nudiflora, Indian laurel (Ficus microcarpa), Coromandel ebony (Diospyros melanoxylon), yellow snake tree (Stereospermum tetragonum), rusty kamala (Mallotus tetracoccus) and Acrocarpus fraxinifolius. During the peak flowering season from January to May, at least 73 species blossom including teak (Tectona grandis), red cedar (Erythroxylum monogynum), hiptage (Hiptage benghalensis), large-flowered bay tree (Persea macrantha), zunna berry (Ziziphus rugosa) and creeping smartweed (Persicaria chinensis). They depend on pollination by giant honey bee, Asiatic honey bee (Apis cerana), red dwarf honey bee (A. florea) and Trigona bees.