The Javan rhinoceros (Rhinoceros sondaicus), also known as the Sunda rhinoceros or lesser one-horned rhinoceros, is a very rare member of the family Rhinocerotidae and one of five extant rhinoceroses. It belongs to the same genus as the Indian rhinoceros, and has similar mosaicked, armour-like skin, but it is smaller (in fact, it is closer in size to the black rhinoceros of the genus Diceros. Only adult males have horns; females lack them altogether. 

Once the most widespread of Asian rhinoceroses, the Javan rhinoceros ranged from the islands of Java and Sumatra, throughout Southeast Asia, and into India and China. The species is critically endangered, with only one known population in the wild, and no individuals in captivity. It is possibly the rarest large mammal on earth, with a population of as few as 58 to 61 in Ujung Kulon National Park at the western tip of Java in Indonesia.

The Javan rhino can live around 30-45 years in the wild. It historically inhabited lowland rain forest, wet grasslands, and large floodplains. It is mostly solitary, except for courtship and offspring-rearing, though groups may occasionally congregate near wallows and salt licks. Aside from humans, adults have no predators in their range.

Scientists and conservationists rarely study the animals directly due to their extreme rarity and the danger of interfering with such an endangered species.

It is the largest animal in Java and the second-largest animal in Indonesia after the Asian elephant. The body length of the Javan rhino (including its head) can be up to 2 to 4 m (6.6 to 13.1 ft), and it can reach a height of 1.4-1.7 m (4.6-5.6 ft). 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. 

Like the Indian rhino, the Javan rhinoceros has a single horn (the other extant species have two horns). Its horn is the smallest of all extant rhinos, usually less than 20 cm (7.9 in) with the longest recorded only 27 cm (11 in).

The main factor in the continued decline of the Javan rhinoceros population has been poaching for horns, a problem that affects all rhino species. The horns have been a traded commodity for more than 2,000 years in China, where they are believed to have healing properties. Historically, the rhinoceros' hide was used to make armor for Chinese soldiers, and some local tribes in Vietnam believed the hide could be used to make an antidote for snake venom.