The hooded seal or bladdernose seal (Cystophora cristata) is a large phocid found only in the central and western North Atlantic, ranging from Svalbard in the east to the Gulf of St. Lawrence in the west. The seals are typically silver-grey or white in color, with black spots that vary in size covering most of the body. Hooded seal pups are known as "blue-backs" because their coats are blue-grey on the back with whitish bellies. This coat is shed after 14 months of age when the pups molt. It is the only species in the genus Cystophora.

The generic name Cystophora means "bladder-bearer" in Greek, from the species' unusual sexual ornament – a peculiar inflatable bladder septum on the head of the adult male. This bladder hangs between the eyes and down over the upper lip in the deflated state. In addition, the hooded seal can inflate a large balloon-like sac from one of its nostrils. This is done by shutting one nostril valve and inflating a membrane, which then protrudes from the other nostril.

Adult males are 2.6 meters (8 ft 6 in) long on average, can grow to 3.5 m, and weigh 300–410 kg (660–900 lb). Sexual dimorphism is obvious from birth and females are much smaller: 2.03 meters (6 ft 8 in) long and weighing 145–300 kg (320–661 lb). The color is silvery; the body is scattered with dark, irregular marks. The head is darker than the rest of the body, and without marks.

Hooded seals live primarily on drifting pack ice and in deep water in the Arctic Ocean and North Atlantic. Although some drift away to warmer regions during the year, their best survival rate is in colder climates. They can be found on four distinct areas with pack ice: near Jan Mayen Island (northeast of Iceland); off Labrador and northeastern Newfoundland; the Gulf of St. Lawrence; and the Davis Strait (off midwestern Greenland). Males appear to be localized around areas of complex seabed, such as Baffin Bay, Davis Strait, and the Flemish cap, while females concentrate their habitat efforts primarily on shelf areas, such as the Labrador Shelf. Hooded seals are known to be a highly migratory species that often wander long distances, as far west as Alaska and as far south as the Canary Islands and Guadeloupe. Prior to the mid 1990s, hooded seal sightings in Maine and the east Atlantic were rare, but began increasing in the mid 1990s. From January 1997 to December 1999, a total of 84 recorded sightings of hooded seals occurred in the Gulf of Maine, one in France and one in Portugal. From 1996 to 2006, five strandings and sightings were noted near the Spanish coasts in the Mediterranean Sea. There is no scientific explanation for the increase in sightings and range of the hooded seal.

The diet of the hooded seal is composed primarily of various amphipods (crustaceans), euphausiids (krill), and fish, including Atlantic argentine, capelin, Greenland halibut, flounder, polar cod, herring, and redfish. They also are known to eat octopus, squid, sea stars, shrimp, other crustaceans, and mussels. Relative to the other species, hooded seals consume 3 times the proportion of redfish; percentages of capelin were similar in relation to closely related species. Capelin is considered a more common choice of sustenance during the winter season. Their diet is considered to be rich in lipids and fatty acids.

Hooded seals tend to feed in relatively deep waters ranging from 100–600 m (330–1,970 ft), and dive from 5 to 25 minute durations. However, some dives can go deeper than 1,016 m (3,333 ft) and as long, or longer, than 52 minutes. Diving is rather continuous, with approximately 90% of their time spent submerged during the day and night, although dives during the day are generally deeper and longer. Dives during the winter are also deeper and longer than those in the summer. It is known that the hooded seal is generally a solitary species, except during breeding and molting seasons. During these two periods, they tend to fast as well. The seals mass annually near the Denmark strait around July, at the time of their molting periods, to mate. Hooded seals are a relatively unsocial species compared to other seals, and they are typically more aggressive and territorial. They demonstrate aggression by inflating the "hood" (which is explained in the "Nasal Cavity" section below). They frequently migrate and remain alone for most of the year, except during mating season.

The hooded seal is known for its uniquely elastic nasal cavity located at the top of its head, also known as the hood. Only males possess this nasal sac, which they begin to develop around the age of four. The hood begins to inflate as the seal makes its initial breath prior to going underwater. It then begins to repetitively deflate and inflate as the seal is swimming. The purpose of this happening is for acoustic signaling, meaning that it occurs when the seal feels threatened and attempts to ward off hostile species when competing for resources such as food and shelter. It also serves to communicate their health and superior status to both other males and females they are attempting to attract. In sexually mature males, a pinkish balloon-like nasal membrane comes out of the left nostril to further aid it in attracting a mate. This membrane, when shaken, is able to produce various sounds and calls depending on whether the seal is underwater or on land. Most of these acoustic signals are used in acoustic situation (about 79%), while about 12% of the signals are used for sexual purposes.

There are four major breeding areas for the hooded seal: the Gulf of St. Lawrence; the "Front" east of Newfoundland; Davis Strait (between Greenland and northern Canada); and the West Ice near Jan Mayen. Male hooded seals are known to have several mates in a single mating season, following the hypothesis that they are polygynous. While some males will defend and mate with just one female for long periods of time, others will be more mobile and tend to mate with multiple females for shorter periods of time, generating maximum offspring within the population. Most males reach sexual maturity by 5 years of age.