The Mallard or wild duck (Anas platyrhynchos), is a dabbling duck which breeds throughout the temperate and subtropical Americas, Europe, Asia, and North Africa, and has been introduced to New Zealand and Australia.

The male birds have a bright green or blue head, while the female's is light brown. The Mallard lives in wetlands, eats water plants, and is gregarious. It is also migratory. The Mallard is the ancestor of all domestic ducks, and can interbreed with other species of genus Anas. However, a potentially terminal side effect of this vast interbreeding capability is gradual genetic dilution, which is causing rarer species of ducks to become at risk for extinction.

The Mallard is 56–65 centimetres (22–26 in) long (of which the body makes up around two-thirds), has a wingspan of 81–98 centimetres (32–39 in), and weighs 0.9–1.2 kilograms (32–42 oz). The breeding male is unmistakable, with a bright bottle-green head, black rear end and a yellowish orange (can also contain some red) bill tipped with black (as opposed to the black/orange bill in females). It has a white collar which demarcates the head from the purple-tinged brown breast, grey brown wings, and a pale grey belly. The dark tail has white borders. The female Mallard is a mottled light brown, like most female dabbling ducks, and has buff cheeks, eyebrow, throat and neck with a darker crown and eye-stripe. However, both the female and male Mallards have distinct purple speculum edged with white, prominent in flight or at rest (though temporarily shed during the annual summer moult). Upon hatching, the plumage coloring of the duckling is yellow on the underside and face (with streaks by the eyes) and black on the backside (with some yellow spots) all the way to the top and back of the head. Its legs and bill are also black but they gradually change color with age. As it nears a month in age, the duckling's plumage becomes drab, looking more like the female (though its plumage is more streaked), but its gender can be distinguished by three factors:

• Bill coloring: Yellow for males, Black and orange for females
• Breast Feathers: Reddish-brown for males, Brown for females
• Tail feathers: Curled for males, Straight for females

After two months of hatching, the fledgling period has ended and the duckling is now a juvenile. It wings will begin to grow and develop flight feathers. During the final period of maturity leading up to adulthood (6–10 months of age), the plumage of female juveniles remains the same while the plumage of male juveniles slowly changes to its recognizable colors. This plumage change also applies to adult Mallard males when they transition in and out of their non-breeding (eclipse) plumage at the beginning and the end of the summer molting period. The average life expectancy of the Mallard is 20 years.

Several species of duck have brown-plumaged females which can be confused with the female Mallard. The female Gadwall (A. strepera) has an orange-lined bill, white belly, black and white speculum which is seen as a white square on the wings in flight, and is a smaller bird.

In captivity, domestic ducks come in wild-type plumages, white, and other colours. Most of these colour variants are also known in domestic Mallards not bred as livestock, but kept as pets, aviary birds, etc., where they are rare but increasing in availability.

A noisy species, the male has a nasal call, and a high-pitched whistle, while the female has a deeper "quack" stereotypically associated with ducks.

The Mallard is a rare example of both Allen's Rule and Bergmann's Rule in birds. Bergmann's Rule, which states that polar forms tend to be larger than related ones from warmer climates, has numerous examples in birds. Allen's Rule says that appendages like ears tend to be smaller in polar forms to minimize heat loss, and larger in tropical and desert equivalents to facilitate heat diffusion, and that the polar taxa are stockier overall. Examples of this rule in birds are rare, as they lack external ears. However, the bill of ducks is very well supplied with blood vessels and is vulnerable to cold.

Due to the malleability of the Mallard's genetic code, which gives it its vast interbreeding capability, mutations in the genes that decide plumage color are very common and have resulted in a wide variety of hybrids such as Brewer's duck (Mallard x Gadwall, Anas strepera)

The Mallard is widely distributed across the Northern Hemisphere, North America from southern and central Alaska to Mexico, the Hawaiian Islands, and across Eurasia, from Iceland and southern Greenland and parts of Morocco (North Africa) in the west, Scandinavia to the north, and to Siberia, Japan, and China in the east. It is strongly migratory in the northern parts of its breeding range, and winters farther south. For example, in North America it winters south to Mexico, but also regularly strays into Central America and the Caribbean between September and May.

The Mallard inhabits a wide range of habitat and climates, from Arctic Tundra to subtropical regions. It is found in both fresh- and salt water wetlands, including parks, small ponds, rivers, lakes and estuaries, as well as shallow inlets and open sea within sight of the coastline. Water depths of less than 1 m (4 ft) are preferred, birds avoiding areas more than a few metres deep. They are attracted to bodies of water with aquatic vegetation.

The Mallard is omnivorous and very flexible in its food choice. Its diet may vary based on several factors, including the stage of the breeding cycle, short term variations in available food, nutrient availability, and inter- and intraspecific competition. The majority of the Mallard's diet seems to be made up of gastropods, invertebrates (including beetles, flies, lepidopterans, dragonflies, and caddisflies), crustaceans, worms, many varieties of seeds and plant matter, and roots and tubers. During the breeding season, male birds were recorded to have eaten 37.6% animal matter and 62.4% plant matter, most notably Echinochloa crus-galli, and nonlaying females ate 37.0% animal matter and 63.0% plant matter, while laying females ate 71.9% animal matter and only 28.1% plant matter. Plants generally make up a larger part of the bird's diet, especially during fall migration and in the winter.

It usually feeds by dabbling for plant food or grazing; there are reports of it eating frogs. It usually nests on a river bank, but not always near water. It is highly gregarious outside of the breeding season and will form large flocks, which are known as a sord.

Mallards usually form pairs (in October and November) only until the female lays eggs at the start of nesting season which is around the beginning of spring (early March to late May), at which time she is left by the male who will join up with other males to await the molting period which begins in June. During the brief time before this, however, the males are still sexually potent and some of them will either remain with the female on standby to sire replacement clutches (for female Mallards that have lost or abandoned their previous clutch) or forcibly mate with females of a different species that appear to be isolated or unattached.

The nesting period can be very stressful for the female; since she lays more than half her body weight in eggs and requires a lot of rest and a feeding/loafing area that is safe from predators. When seeking out a suitable nesting site, the female's preferences are on areas that are well concealed, are inaccessible to ground predators, and/or have few to no predators living nearby. This unfortunately includes urban areas that have roof gardens, enclosed courtyards, and flower boxes on window ledges/balconies more than one story up which prevents the ducklings from leaving safely or at all without human intervention. The clutch is 8–13 eggs, which are incubated for 27–28 days to hatching with 50–60 days to fledgling. The ducklings are precocial and fully capable of swimming as soon as they hatch. However, Filial imprinting will compel them to instinctively stay near the mother not only for warmth and protection but also to learn about and remember their habitat as well as how and where to forage for food. When ducklings mature into flight-capable juveniles, they will learn about and remember their traditional migratory routes (unless they are born and raised in captivity). After this, they will either continue staying with the mother (until the breeding season arrives) or set off on their own to seek out new sources of food and water, both natural and artificial.

When they pair off with mating partners, often one or several drakes will end up "left out". This group will sometimes target an isolated female duck, even when she's of a different species, and proceed to chase and peck at her until she weakens, at which point each male will take turns copulating with the female. Lebret (1961) calls this behaviour ‘Attempted Rape Flight’ and Cramp & Simmons (1977) speak of ‘rape-intent flights’. Male Mallards will also occasionally chase other male ducks of a different species, and even each other, in the same way. In one documented case of ‘homosexual necrophilia’, a male Mallard copulated with another male he was chasing after the chased male died upon flying into a glass window.

Mallards are opportunistically targeted by brood parasites, occasionally having eggs laid in their nests by Redheads, Ruddy Ducks, Lesser Scaup, Gadwalls, Northern Shovelers, Northern Pintails, Cinnamon Teal, Common Goldeneyes, and other Mallards. These eggs are generally accepted when they resemble the eggs of the host Mallard, although the hen may attempt to eject them or even abandon the nest if parasitism occurs during egg laying.