The hooded seal (Cystophora cristata) is an arctic pinniped 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 generic name means "bladder-bearer" in Greek, from the peculiar inflatable bladder on the head of the adult male – this bladder hangs between his eyes and down over his upper lip in the deflated state. In addition, he can inflate a large balloon-like sac from one of his nostrils. This is done by shutting one nostril valve and inflating a membrane which then protrudes from the other nostril.

The bulge develops when the seal is four years old. The male can blow up this bulge, so that it is the size of its head.

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