Dang District (Nepali: दाङ जिल्ला, [d̪äŋ]) is a district of Lumbini Province located in the Inner Terai Rapti Zone of midwestern Nepal. Dang Valley is the first largest valley in Asia and the second largest in the world. It is surrounded by the Sivalik Hills and the Mahabharata Range. Ghorahi is the capital of the Dang district and Deukhuri of the Dang district is capital of the Lumbini province. The district headquarter Ghorahi is the seventh largest city and also the Lumbini province headquarters the largest sub-metropolitan city of Nepal. Tulsipur sub-metropolitan city, the second largest city of Dang, is a major transportation hub with extensive road and air networks. The district covers an area of 2,955 km² and has a population of 674,993 (2021 census).

Dang district has been archaeologically studied extensively since the 20th century due to the discoveries of ancient fossils of apes and early humans. The district is considered the center of Sanskrit language in Nepal and is home to Nepal's second oldest university, Nepal Sanskrit University, which is the only Sanskrit university of the country, as well as Rapti Academy of Health Sciences (RAHS), the state-owned medical college of Lumbini Province with the Prime Minister of Nepal as the Chancellor. Historically, the valley of Dang was a prominent pilgrim place of the Nath Hindu monastic movement, and hence was also famous as 'the land of the Gorakhnath yogis'. The district has numerous temples and gumbas with puranic legendary connections to Mahābhārata, Shaivism, Shaktism and Gorakhnath, making it one of the richest cultural sites of the country.

Archeologists consider the Churiya range very ancient with the existence of Sivapithecus (syn:Ramapithecus), a link between man and ape. The pre-historic study of Dang valley has been carried out by Tribhuvan University since 1966, including the geological study of the Valley by Robert M. West from the American Museum of Natural History and the Department of Mines of then His Majesty's Government of Nepal from 1976, as well as the Paleolithic study of Dang by University of Erlangen-Nuremberg (Fredrich-Alexander-Universität) of Germany in 1984, among others. According to these concurrent researches, Dang valley was a lake approximately 2.5 to 1 million years ago.

During 1984–1986, German researcher Gudrun Corvinus studied the miocene-pliocene stratigraphy and geology of Dang valley researching on the pleistocene holocene period of ancient human settlements. She studied the exposed section of the road being constructed between Koilabas and the Dang Valley running through Deukhuri. Hand axes and other artifacts dated to early Paleolithic (1.8 million to 100,000 years ago) have been found in alluvial deposits along the Babai River in Dang Valley. Archeologists classify these as Acheulean, i.e. 'second-generation' toolmaking that succeeds the very oldest Olduwan.

There are more numerous less ancient archeological sites dating to the Upper Paleolithic/Late Pleistocene (about 50,000 to 10,000 years ago). These are also along the Babai, as well as in Deukhuri Valley (Rapti River) adjacent and south of Dang Valley. Archeologists classify these as Acheulean, i.e. 'second-generation' toolmaking that succeeds the very oldest Olduwan.

Since ancient times, Dang valley is considered to have been inhabited by the indigenous Tharu people, albeit the exact timing of origin is still a matter of archeological research Researchers consider that the valley was a growing center of Tharu civilisation. In the current times, Tharu people comprise one of the prominent indigenous ethnic communities of Nepal, and the Tharus inhabiting Dang valley are called Dangaura Tharus who have been able to retain their rich and unique traditions.

With a long history of over 1300 years, the valley is recognised to have been a focal pilgrimage and shelter place for the disciples of Nath sect, the followers of Hindu yogi and mahasiddha Gorakhnath and his guru Matsyendranath. The Goraksha or Ratnanath temple in the district is believed to have been built by king-turned-disciple Ratnanath, whose original name was King Ratnaparikshak and was given initiation by Gorakhnath himself in the nearby forest.

Evidence show that when the Khasa Kingdom was in existence in western Nepal, Dang was virtually under their political domination. One of the document of Punya Malla mentions that the valley was awarded to Jayakar Pundit as a virta in 1336 AD. From 1350 AD onwards, Meghraj Singh Chauhan of Chauhan dynasty is recorded to have been the king of the region as per the records in the Yogi Ratna Nath's Chaughera monastery of Dang valley, the third-in-line of Yogi Gorakhnath. The temple held a strong spiritual importance over the rulers of the valley, even after the change of power to new dynasties as such the internal activities of the temple were almost entirely unaffected for hundreds of years. Nevills Gazetteer of Bahreich (1922:124) mentions that in 1485 AD, Dangdun was in the possession of Udat Singh. Evidence shows that the last independent king Nawal Singh Chauhan ruled from Chaughera of Dang until 1760 AD as the House of Tulsipur which ruled one of the largest Taluqs of Oudh, India, which then included the Dang and Deukhuri Valleys.The town shares its name with another Tulsipur in Dang Deukhuri District, Nepal (c.65k North); the two towns are linked historically by having the same ruler.