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The Virginia opossum (Didelphis virginiana), also known as the North American opossum, is a member of the opossum family found from southern Canada to northern Costa Rica (making it the northernmost marsupial in the world). Commonly referred to simply as the possum, it is a solitary nocturnal animal about the size of a domestic cat, and a successful opportunist.

Opossums are familiar to many North Americans as they frequently inhabit settled areas near food sources like trash cans, pet food, compost piles, gardens or housemice. Their slow, nocturnal nature and their attraction to roadside carrion make opossums more likely to become roadkill.

The Virginia opossum is the original animal named "opossum", a word which comes from Algonquian wapathemwa, meaning "white animal". Colloquially, the Virginia opossum is frequently just called a "possum". The term is applied more generally to any of the other marsupials of the families Didelphidae and Caenolestidae. The generic name (Didelphis) is derived from Ancient Greek: di, "two", and delphus, "womb".

The possums of Australia, whose name derives from their similarity to the American species, are also marsupials, but of the order Diprotodontia.

The Virginia opossum is known in Mexico as tlacuache, tacuachi, and tlacuachi, from the Nahuatl word tlacuatzin.

Virginia opossums can vary considerably in size, with larger specimens found to the north of the opossum's range and smaller specimens in the tropics. They measure 33–55 cm (13–22 in) long from their snout to the base of the tail, with the tail adding another 25–54 cm (9.8–21.3 in). Males are slightly larger, with an average body length of 40.8 cm (16.1 in) with an average tail length of 29.4 cm (11.6 in), while females are 40.6 cm (16.0 in) long with a 28.1 cm (11.1 in) tail. Weight for males ranges from 2.1–2.8 kg (4.6–6.2 lb) and for females from 1.9–2.1 kg (4.2–4.6 lb).

Their coats are a dull grayish brown, other than on their faces, which are white. Opossums have long, hairless, prehensile tails, which can be used to grab branches and carry small objects. They also have hairless ears and a long, flat nose. Opossums have 50 teeth, more than any other North American land mammal, and opposable, clawless thumbs on their rear limbs. Opossums have 13 nipples, arranged in a circle of 12 with one in the middle.

The dental formula of an opossum is (5.1.3.4/4.1.3.4). No other mammal in North America has more than 6 upper incisors, but the Virginia opossum has 10.

Perhaps surprisingly for such a widespread and successful species, the Virginia opossum has one of the lowest encephalization quotients of any marsupial. Its brain is one-fifth the size of a raccoon's.

Opossums are omnivorous (sometimes said to be insectivorous) and eat a wide range of plant-based food, as well as animal-based food like small invertebrates, carrion, eggs, fish, amphibians, reptiles, birds, small mammals, and other small animals.

Insects such as grasshoppers, crickets, and beetles make up the bulk of the animal foods eaten by opossums. It has been stated that opossums eat up to 95% of the ticks they encounter and may eat up to 5,000 ticks per season, helping to prevent the spread of tick-born illnesses, including Lyme disease and Rocky Mountain spotted fever. This interpretation has been challenged. A widely publicized 2009 study by the Cary Institute indicated that Virginia opossums in a laboratory setting could eat thousands of ticks per week grooming. However, subsequent studies of the stomach contents of wild Virginia opossums have not found any ticks in their diet.

Small animals include young rabbits, meadow voles, mice, rats, birds, snakes, lizards, frogs, fish, crayfish, gastropods, and earthworms. The Virginia opossum has been found to be very resistant to snake venom. Attracted to carrion on the side of the highway, opossums are at an increased risk of being hit by motor vehicles.