Silent Valley National Park, National park in Kerala, India

Silent Valley National Park is a protected area in the Nilgiri hills of Kerala state, covering roughly 89 square kilometers (34 square miles) of unbroken tropical rainforest. The area ranges in elevation from 900 to 2,300 meters (2,950 to 7,545 feet) and shelters many mammal species, including lion-tailed macaques, Bengal tigers, and Asian elephants.

The region gained protected status in 1984 after nationwide demonstrations stopped a dam project that would have flooded large sections of the forest. The movement is now remembered as one of India's first successful environmental campaigns and helped raise awareness about preserving tropical ecosystems.

Local people call the forests Sairandhri, a name also used for Panchali, the Mahabharata heroine. The Kunthipuzha, the only river flowing through the forest, is tied to these legends. The park's name has three explanations: the absence of noisy cicadas, an anglicisation of Sairandhri, or the lion-tailed macaque's scientific name, Macaca silenus.

The park is reached via Mannarkkad in Palakkad district; the visitors' centre is at Sairandhri. The only river flowing through Sairandhri forest is the Kunthipuzha. Annual rainfall ranges from 2,800 to 3,400 mm, and the eastern parts lie in the Western Ghats rain shadow, so they receive less rain.

The park protects one of the last populations of the lion-tailed macaque, a monkey species found only in the Western Ghats and easily recognized by its silvery mane. Researchers have documented more than 400 bird species here, more than in most other Indian protected areas of similar size.