The emu (Dromaius novaehollandiae) is the largest bird native to Australia and the only extant member of the genus Dromaius. It is the second-largest extant bird in the world by height, after its ratite relative, the ostrich. There are three subspecies of emus in Australia. The emu is common over most of mainland Australia, although it avoids heavily populated areas, dense forest and arid areas.

The soft-feathered, brown, flightless birds reach up to 1.9 metres (6.2 ft) in height. They have long thin necks and legs. Emus can travel great distances at a fast, economical trot and, if necessary, can sprint at 50 km/h (31 mph). Their long legs allow them to take strides of up to 275 centimetres (9.02 ft) They are opportunistically nomadic and may travel long distances to find food; they feed on a variety of plants and insects, but have been known to go for weeks without food. Emus ingest stones, glass shards and bits of metal to grind food in the digestive system. They drink infrequently, but take in copious fluids when the opportunity arises. Emus will sit in water and are also able to swim. They are curious birds who are known to follow and watch other animals and humans. Emus do not sleep continuously at night but in several short stints sitting down.

Emus use their strongly clawed feet as a defence mechanism. Their legs are among the strongest of any animal, allowing them to rip metal wire fences. They are endowed with good eyesight and hearing, which allows them to detect predators in the vicinity. The plumage varies regionally, matching the surrounding environment and improving its camouflage. The feather structure prevents heat from flowing into the skin, permitting emus to be active during the midday heat. They can tolerate a wide range of temperatures and thermoregulate effectively. Males and females are hard to distinguish visually, but can be differentiated by the types of loud sounds they emit by manipulating an inflatable neck sac. Emus breed in May and June and are not monogamous; fighting among females for a mate is common. Females can mate several times and lay several batches of eggs in one season. The animals put on weight before the breeding season and the male does most of the incubation, losing significant weight during this time as he does not eat. The eggs hatch after around eight weeks, and the young are nurtured by their fathers. They reach full size after around six months, but can remain with their family until the next breeding season half a year later. Emus can live between 10 and 20 years in the wild and are predated by dingos, eagles and hawks. They can jump and kick to avoid dingos, but against eagles and hawks they can only run and swerve.

The Tasmanian emu and King Island emu subspecies that previously inhabited Tasmania and King Island became extinct after the European settlement of Australia in 1788 and the distribution of the mainland subspecies has been influenced by human activities. Once common on the east coast, emus are now uncommon there; by contrast, the development of agriculture and the provision of water for stock in the interior of the continent have increased the range of the emu in arid regions and it is of Least Concern for conservation. They were a food and fuel source for indigenous Australians and early European settlers. Emus are farmed for their meat, oil and leather. Emu is a lean meat and while it is often claimed by marketers that the oil has anti-inflammatory and anti-oxidative effects this has not been scientifically verified in humans.

The emu is an important cultural icon of Australia, appearing on the coat of arms and various coins. The bird features prominently in Indigenous Australian mythology and hundreds of places are named after it.

There are reports the emu was first sighted by European explorers in 1696 when they made a brief visit to the coast of Western Australia. It was thought to have been spotted on the east coast of Australia before 1788 when the first European settlement occurred. It was first described under the name of the "New Holland cassowary" in Arthur Phillip's Voyage to Botany Bay, published in 1789. The species was named by ornithologist John Latham on a specimen from the Sydney, Australia area, which was referred to as New Holland at the time. He collaborated on Phillip's book and provided the first descriptions of and names for many Australian bird species; its name is Latin for "fast-footed New Hollander". The etymology of the common name "emu" is uncertain, but is thought to have come from an Arabic word for large bird that was later used by Portuguese explorers to describe the related cassowary in Australia and New Guinea. Another theory is that it comes from the word "ema", which is used in Portuguese to denote a large bird akin to an ostrich or crane. In Victoria, some terms for the emu were Barrimal in the Dja Dja Wurrung language, myoure in Gunai, and courn in Jardwadjali. It was known as murawung or birabayin to the local Eora and Darug inhabitants of the Sydney basin.

In his original 1816 description of the emu, Vieillot used two generic names; first Dromiceius, then Dromaius a few pages later. It has been a point of contention ever since which is correct; the latter is more correctly formed, but the convention in taxonomy is that the first name given stands, unless it is clearly a typographical error. Most modern publications, including those of the Australian government, use Dromaius, with Dromiceius mentioned as an alternative spelling.

Emus are large birds. The largest can reach up to 1.5 to 1.9 m (4.9–6.2 ft) in height, 1 to 1.3 m (3.3–4.3 ft) at the shoulder. In length measured from the bill to the tail, emus range from 139 to 164 cm (55 to 65 in), with males averaging 148.5 cm (58.5 in) and females averaging 156.8 cm (61.7 in). Emus weigh between 18 and 60 kg (40 and 132 lb), with an average of 31.5 and 36.9 kg (69 and 81 lb) in males and females, respectively. Females are usually larger than males by a small amount, and are substantially wider across the rump.

They have small vestigial wings, the wing chord measuring around 20 cm (7.9 in) long, and have a small claw at the tip of the wing. The bill is quite small, measuring 5.6 to 6.7 cm (2.2 to 2.6 in). The emu flaps its wings when it is running and it is believed that they stabilise the bird when it is moving. It has a long neck and legs. Their ability to run at high speeds, 48 km/h (30 mph), is due to their highly specialised pelvic limb musculature. Their feet have only three toes and a similarly reduced number of bones and associated foot muscles; they are the only birds with gastrocnemius muscles in the back of the lower legs. The pelvic limb muscles of emus have a similar contribution to total body mass as the flight muscles of flying birds. When walking, the emu takes strides at every 100 cm (3.3 ft), but at full gallop, a stride can be as long as 275 cm (9.02 ft). Its legs are devoid of feathers and underneath its feet are thick, cushioned pads. Like the cassowary, the emu has sharp claws on its toes which are its major defensive attribute. This is used in combat to inflict wounds on opponents by kicking. The toe and claw are a total of 15 centimetres (5.9 in). They have a soft bill, adapted for grazing.

The emu has good eyesight and hearing, which allows it to detect nearby threats. Its legs are among the strongest of any animals, powerful enough to tear down metal wire fences.

The neck of the emu is pale blue and shows through its sparse feathers. They have brown to grey-brown plumage of shaggy appearance; the shafts and the tips of the feathers are black. Solar radiation is absorbed by the tips, and the loose-packed inner plumage insulates the skin. The resultant heat is prevented from flowing to the skin by the insulation provided by the coat, allowing the bird to be active during the heat of the day. A unique feature of the emu feather is its double rachis emerging from a single shaft. Both of the rachis have the same length, and the texture is variable; the near the quill it is rather furry, but the external ends resemble grass. The sexes are similar in appearance, although the male's penis can become visible when it defecates. The plumage varies in colour due to environmental factors, giving the bird a natural camouflage. Feathers of emus in more arid area with red soil have a similarly tinted plumage but are darker in animals residing in damp conditions.

The eyes of an emu are protected by nictitating membranes. These are translucent, secondary eyelids that move from the end of the eye closest to the beak to cover the other side. This is used by the emu as a protective visor to protect its eyes from dust that is prevalent in windy and arid deserts. The emu also has a tracheal pouch, which becomes more prominent during the mating season. It is often used during courting, and it has speculated that it is used for communication on a day-to-day basis. The pouch is more than 30 centimetres (12 in), is spacious and the wall in very thin. Its opening's width is only 8 centimetres (3.1 in). The quantity of air that goes through the pouch, as determined by the emu deciding to open or close it, affects the pitch of an emu's call. Females typically cry more loudly than males.

On very hot days, emus pant to maintain their body temperature, their lungs work as evaporative coolers and, unlike some other species, the resulting low levels of carbon dioxide in the blood do not appear to cause alkalosis. For normal breathing in cooler weather, they have large, multifolded nasal passages. Cool air warms as it passes through into the lungs, extracting heat from the nasal region. On exhalation, the emu's cold nasal turbinates condense moisture back out of the air and absorb it for reuse. As with other ratites, the emu has great homeothermic ability, and can maintain this status from −5 to 45 °C. The thermoneutral zone of emus lies between 10–15 and 30 °C.

As with other ratites, the emu has a relatively low rate of metabolism compared to other types of birds, but the rate depends on activity, especially due to resulting changes to thermodynamics. At −5 degrees, the metabolism rate of an emu while sitting down is around 60% of the value for one that is standing, as the lack of feathers under its stomach leads to a higher rate of heat loss when it is standing up and exposing the underbelly.