The Pampas Deer (Ozotoceros bezoarticus) is a species of deer that live in the grasslands of South America at low elevations. They are known as veado-campeiro in Portuguese and as venado or gama in Spanish. It is the only species in the genus Ozotoceros.

Their habitat includes water and hills, often with winter drought, and grass that is high enough to cover a standing deer. Many of them live on the Pantanal wetlands, where there are ongoing conservation efforts, and other areas of annual flooding cycles. Human activity has changed much of the original landscape.

They are known to live up to 12 years in the wild, longer if captive, but are threatened due to over-hunting and habitat loss. Many people are concerned over this loss, because a healthy deer population means a healthy grassland, and a healthy grassland is home to many species, some also threatened. Many North American birds migrate south to these areas, and if the Pampas deer habitat is lost, they are afraid these bird species will also decline. There are approximately 80,000 Pampas deer total, with the majority of them living in Brazil.

Fossil records indicate that New World deer traveled to South America from North America as part of the Great American Interchange around 2.5 million years ago, following the formation of the Isthmus of Panama. They rapidly evolved into different species, with only a few surviving today. Due to the large continental glaciers and the high soil acidity in areas where there were no glaciers, a huge part of the fossil record has been destroyed, so there is no indication of what these early New World deer looked like. Fossil records begin with clear differentiation and are close to what they look like now. The Pampas deer evolved as plains dwellers; their direct ancestor first appeared during the Pleistocene epoch.

The deer may have evolved without culling predators[clarification needed] because when alarmed, they stamp their feet, have a particular trot and whistle, and deposit odor. Pampas deer have a similar gene pattern to the related marsh deer of the genus Blastocerus, having two fused chromosomes.

There are five recognised subspecies:

• O. b. bezoarticus - eastern and central Brazil, south of the Amazon river into Uruguay

• O. b. arerunguaensis - northwestern Uruguay

• O. b. celer - southern Argentina

• O. b. leucogaster - southwestern Brazil, southeastern Bolivia, Paraguay, and northern Argentina

• O. b. uruguayensis - eastern Uruguay

Pampas deer are among the most genetically polymorphic mammals. Their current high nucleotide diversity shows that they had very large numbers in the recent past.

Pampas deer have tan fur, lighter on their undersides and insides of legs. Their coats do not change with the seasons. They have white spots above their lips and white patches on their throats. Their shoulder height is 60–65 cm (24–26 in) in females and 65–70 cm (26–28 in) in males. Their tails are short and bushy, 10 cm to 15 cm long, and when they run, they lift their tail to reveal a white patch, just like white-tailed deer.

Adult males typically weigh 24–34 kg (53–75 lb), but have been documented up to 40 kg (88 lb), and females typically weigh 22–29 kg (49–64 lb). They are a small species of deer, with relatively little sexual dimorphism. Males have small, lightweight antlers that are 3-pronged, which go through a yearly cycle of shedding in August or September, with a new grown set by December. The lower front main prong of the antlers is not divided, but the upper prong is. Females have hair whorls that look like tiny antlers stubs. Females and males have different stances during urination. Males have a strong smell secreted from glands in their back hooves that can be detected up to 1.5 km away. Compared to other small ruminants, the males have small testicles relative to their body size.

In Argentina, the mating season is December to February. In Uruguay, the mating season is February to April. Courtship behavior is submissive, such as low stretching, crouching, and turning away. The male initiates courtship with a low stretch. He makes a soft buzzing sound. He nuzzles the female and may flick his tongue at her, and averts his eyes. He stays near her, and may follow her for a long time, smelling her urine. Sometimes the female responds to courtship by lying on the ground.

Pampas deer do not defend territory or mates, but do have displays of dominance. They show dominance by keeping their heads up and trying to keep their side forward, and use slow, deliberate movements. When bucks are challenging each other, they rub their horns into vegetation and scrape them on the ground. They may urinate into the scrape they've made, and sometimes defecate. They rub the scent glands on their heads and faces into plants and objects. They usually do not fight, but just spar with each other, and they do commonly bite. Sparring is initiated by the smaller buck touching noses with the larger buck. Groups are not separated by sex, and bucks will drift between groups. There are usually only 2-6 deer in a group, but there can be many more in good feeding areas. They do not have monogamous pairs, nor are there harems.

When they feel they may be in danger, they hide low in the foliage and hold, and then bound off about 100–200 meters, often looking back at the disturbance. Because they bound in long flat jumps and have not been observed to run, they are not thought to be endurance runners. If they are alone, they may just quietly slip away. Females with a fawn will fake a limp to distract a predator, or if they are unsure of a situation, such as if a human appears.

They will often stand on their hind legs to reach food or see over something. They are sedentary, with no seasonal or even daily movements. They usually feed regularly during the day, but sometimes have nocturnal activity. The Pampas deer are very curious and like to explore. Although this is endearing to observers, their lack of fleeing at the sight of humans makes them easier for poachers to kill.

Pampas deer have been seen eating new green growth, shrubs, and herbs. Most of the plant life they consume grows in moist soils. To see if Pampas deer compete with cattle for food, their feces were studied and compared to cattle feces. They do in fact eat the same plants, but in different proportions. The pampas deer eat less grass and more forbs (flowering broad leafed plants with soft stems) and browse (shoots, leaves, and twigs), respectively. During the rainy season, 20% of their diet consists of new grasses. They will move with the availability of food, particularly the flowering plants. The presence of cattle increases the amount of sprouting grass, which is preferred by Pampas deer, furthering the idea that the deer do not compete with cattle for food. Opposing research shows that Pampas deer avoid areas inhabited by cattle, and when cattle are absent have much larger home ranges.

The Pampas deer of southern Argentina, once very abundant, are now considered a threatened species by the IUCN. The IUCN separates the subspecies O. b. celer in southern Argentina as endangered. The diseases that particularly plague O. b. celer are gut parasites and foot and mouth diseases. Their overall decline is also due (in part) from hunting and poaching, but also from agricultural habitat loss (thus exposing deer to diseases from domesticated and feral livestock), competition from more recently-introduced wildlife, and general over-exploitation. There is less than 1% of their natural habitat left that was present in 1900. The deer in Argentina and Uruguay have fewer natural predators. They used to be the prey of cougars and many more jaguars. Those in Brazil still have the felines to fear. Some areas with low population are easily attributed to poaching, due to a sudden lower number of deer in an area. In the mid-1970s, ten individuals, out of a group of 16 located in Punta Medanos, were killed by poachers. The rest were extirpated by extensive human activity. Lack of funding and technology have made it difficult for biologists to track and aid the deer population, while donations and grants from organizations and universities in the United States have helped immensely with the situation. In 1975, there were less than 100 of the subspecies O. b. celer, but by 1980, there were around 400. The population has been continuing to increase, although not at an incredibly fast pace. One of the discrepancies is simply the fact that, later on, previously unknown subspecies and groups were discovered.