Up until the mid-19th to about the early 20th century, the Javan rhinoceros had ranged beyond the islands of Java and Sumatra and onto the mainland of Southeast Asia and Indochina, northwest into East India, Bhutan, and the south of China.

It is among the rarest large mammals on the planet Earth,[5]: 21  with a population of approximately 74 rhinos within Ujung Kulon National Park, at the far western tip of Java, Indonesia.

[8] The decline of the Javan rhinoceros is primarily attributed to poaching, for the males' horns, which—despite merely being composed of keratin—are highly valued in traditional Chinese medicine, fetching as much as US$30,000 per kg on the black market.

Loss of habitat and massive human population growth (especially post-wartimes, such as the Vietnam War) have also contributed to its decline and hindered the species' recovery.

Two adult female Javan rhinoceroses, each with a calf, were filmed using a motion-triggered trail camera, the video being released on 28 February 2011 by WWF and Indonesia's National Park Authority, proving they are still breeding in the wild.

[11] In April 2012, the National Parks Authority released further trailcam videos showing 35 individuals, including mother-offspring pairs and courting adults.

A cladogram showing the relationships of recent and Late Pleistocene rhinoceros species (minus Stephanorhinus hemitoechus) based on whole nuclear genomes, after Liu et al., 2021.

Adults are variously reported to weigh between 900 and 2,300 kg (2,000 and 5,100 lb), although a study to collect accurate measurements of the animals has never been conducted and is not a priority because of their extreme conservation status.

Javan rhinos do not appear to often use their horn for fighting but instead uses it to scrape mud away in wallows, to pull down plants for eating, and to open paths through thick vegetation.

[30][31] The animal was once widespread from Assam and Bengal (where their range would have overlapped with both the Sumatran and Indian rhinos)[32] eastward to Myanmar, Thailand, Cambodia, Laos, Vietnam, and southwards to the Malay Peninsula and the islands of Sumatra, Java, and Borneo.

[33][34] Javan rhinoceros remains were also found at the Neolithic site of Hemudu in Zhejiang, China, and the Classic of Mountains and Seas appears to describe one living in the Yangtze River basin.

[35] The Javan rhino primarily inhabits dense, lowland rain forests, grasslands, and reed beds with abundant rivers, large floodplains, or wet areas with many mud wallows.

Starting around 1000 BC, the northern range of the rhinoceros extended into China, but began moving southward at roughly 0.5 km (0.31 mi) per year, as human settlements increased in the region.

Local hunters and woodcutters in Cambodia claim to have seen Javan rhinos in the Cardamom Mountains, but surveys of the area have failed to find any evidence of them.

Wallowing in mud is a common behavior for all rhinos; the activity allows them to maintain cool body temperatures and helps prevent disease and parasite infestation.

[9] Nevertheless, when humans approach too closely, the Javan rhino becomes aggressive and will attack, stabbing with the incisors of its lower jaw while thrusting upward with its head.

[30] The Javan rhinoceros is herbivorous, eating diverse plant species, especially their shoots, twigs, young foliage and fallen fruit.

The salt licks common in its historical range do not exist in Ujung Kulon but the rhinos there have been observed drinking seawater, likely for the same nutritional need.

[41] The main factor in the continued decline of the Javan rhinoceros population has been poaching for horns, a problem that affects all rhino species.

[43] Because the rhinoceros' range encompasses many areas of poverty, it has been difficult to convince local people not to kill a seemingly (otherwise) useless animal which could be sold for a large sum of money.

[5]: 31 Loss of habitat because of agriculture has also contributed to its decline, though this is no longer as significant a factor because the rhinoceros only lives in one nationally protected park.

[31] In 1931, as the Javan rhinoceros was on the brink of extinction in Sumatra, the government of the Dutch East Indies declared the rhino a legally protected species, which it has remained ever since.

Although the rhinos in Ujung Kulon have no natural predators, they have to compete for scarce resources with wild cattle, which may keep their numbers below the peninsula's carrying capacity.

Comparing the acid insoluble ash (MA) content of faeces and in the dry weight of food provided reliable estimates of digestibility, and this method has potential for wider application in situations where total collection of faecal matter is not feasible.

Overall energy consumption was related to the size of the animal, while the digestibility of plants consumed appeared to be influenced by individual age and habitat conditions.

[50] In May 2017, Director of the Biodiversity Conservation at the Ministry of Environment and Forestry, Bambang Dahono Adji announced plans to transfer the rhinos to the Cikepuh Wildlife Sanctuary located in West Java.

[51] The animals will first undergo DNA tests to determine lineage and risk to disease so as to avoid issues such as "inbreeding" or marriage kinship.

The combat wrought havoc on the ecosystems of the region through the use of napalm, extensive defoliation from Agent Orange, aerial bombing, use of landmines, and overhunting by local poachers.

[43] By the early 2000s, their population was feared to have declined past the point of recovery in Vietnam, with some conservationists estimating as few as three to eight rhinos, and possibly no bulls, survived.

[64][65] Another rhinoceros carving in the centre of a circular arrangement in a column with other circles containing elephants and water buffalo is known from the temple of Ta Prohm.