Puffer, also called swellfish, or blowfish, any of about 90 species of fishes of the family Tetraodontidae, noted for their ability when disturbed to inflate themselves so greatly with air or water that they become globular in form. Puffers are found in warm and temperate regions around the world, primarily in the sea but also, in some instances, in brackish or fresh water. They have tough, usually prickly skins and fused teeth that form a beaklike structure with a split in the centre of each jaw. The largest puffers grow about 90 centimetres (3 feet) long but most are considerably smaller.

Many species are poisonous; a highly toxic substance, tetraodontoxin, is especially concentrated in the internal organs. Although this substance can cause death, puffers are sometimes used as food. In Japan, where the fishes are called fugu, they must be carefully cleaned and prepared by a specially trained chef.

Related to the puffers are about 12 species, also capable of inflating themselves, known as the sharp-nosed puffers. These fishes, which comprise the genus Canthigaster and the family Canthigasteridae, are found throughout the world. They are small fishes with rather long, pointed snouts and, unlike the puffers, inconspicuous nostrils. They are brightly coloured and no more than about 20 centimetres long. Like some puffers, they are sometimes kept in marine aquariums.