The sparse fossil data we have suggest that water deer had a historic range much larger than it is today. According to Endi Zhang in his 1993 Ph.D. thesis, ancient Chinese texts suggest water deer were a common animal that roamed over much of China, widespread at latitudes between 28 and 42 degrees N and between a longitude of about 111 degrees E and the western coast of the Pacific. Since then, water deer remains have been found at archaeological sites that have extended their historic range even further. Yen-Jean Chen and colleagues reported, for example, remains from Iron Age sites in Taichung, west-central Taiwan during 2016 and suggested that they became extinct the early 19th Century, probably in response to human expansion into the wetlands. More recently, palaeontologist Chris Stimpson and his team found water deer jaw fragments among remains in a cave at Tràng An in north-east Vietnam dating to between 13,000 and 16,000 years before the present.
Taking archaeological and contemporary records together, the species could be found from central China in the west, east across to the East China Sea coast, island of Taiwan and the eastern coast of South Korea, south to at least northern Vietnam, and north into north-east China and possibly south-east Mongolia. There are even remains from southern Japan that date to the Middle Pleistocene, about half a million years ago.
In their 2006 paper to Biochemical Genetics, Jie Hu and colleagues present a map showing a considerable reduction in the current distribution compared with its documented historical range, describing the population declines as “drastic”. Apparently wiped out of Qingpu and Fengxian in the early 1920s, their range in China has been contracting eastwards over past five or six decades into isolated populations the coastal areas of Jiangsu Province, Poyang Lake areas in Jiangxi Province, and Zhoushan Archipelago of Zhejiang Province. Composed of 1,384 islets, Zhoushan being the largest, the Zhoushan Islands group is located just outside the Hangzhou Bay in the East China Sea. The earliest record of water deer in the Zhoushan Archipelago of which I'm aware is a reference in The Annals of Dinghai County, published in 1923.
While poaching remains a threat to the species' continued survival, there has been some good news. A recent project to reintroduce water deer to Shanghai has been largely successful, while several photos and videos of water deer, including a fawn, collected using trailcams set in Jilin Baishan Musk Deer National Nature Reserve, north-east China, during 2018 represent the first confirmed report of deer in the province since 1949.
In China, most texts show the remaining population having a southern boundary of roughly Fujian and Guangdong and Guangxi provinces, stretching north into Jiangsu and Anhui provinces, west as far as Hubei and Hunan provinces, and along the eastern Yangtze basin, with Poyang Lake being a principal population. In their Chinese text on the changes to plants and animals in China through recent history, published in 2006, Huanran and Rongsheng Wen show the species as being widely distributed throughout Guangxi in the south, but only sporadically present in the northern part of Guangdong Province. A more recent analysis of survey data, including new transects and camera trapping suggests some recolonisation of north-eastern areas. The data, collected by a team at the Northeast Forestry University in Harbin, led by Zongzhi Li, was published in Scientific Reports during 2023 and records water deer in eastern areas of Jilin and Heilongjiang provinces.
In Korea, akin to the situation in China, the species' distribution may have been substantially reduced through poaching and habitat destruction, although little information is available. In their 2015 review of water deer distribution for the IUCN Red List of Threatened Species, Richard Harris and Will Duckworth note that the species is reported as being “relatively widespread” in the Republic of Korea (South), particularly along the west coast, and apparently also in the lower lying parts of Democratic People's Republic of Korea (North), although they note that assessing the true status is confounded by repeated reports of widespread and frequent releases of captive-bred stock. In a 2011 paper to Landscape Ecology and Engineering, Ewha Womans University researcher Baek-Jun Kim and colleagues report that water deer seemed to occur in most areas of South Korea except for Seoul and Jeju Province and were even occasionally observed in, but tended to avoid, Kyunggi Province, which is near Seoul and one of the most actively developed areas. They appear absent from the off-shore Korean islands of Ulleung-do, Dok-do and Jeju-do.
In summary, this species is native to central China's eastern Yangtze Valley, parts of eastern China, the Zhoushan Island archipelago off coast of Zhejiang Province, the Republic of Korea, and Democratic People's Republic of Korea – throughout the wetlands and forests of the Civilian Controlled Zone (CCZ) and the Demilitarized Zone (DMZ) that divides the two countries. In North Korea water deer inhabit the Taebak and Nagrim mountains, Kangwon Province, and adjacent South Hamgyong Province, while in South Korea it is more widely distributed, occurring in all provinces except Seoul and Jeju. In recent years the population appears to have expanded from North Korea into south-west Russia and potentially repopulating areas of eastern Jilin and Heilongjiang provinces.