The Dall’s porpoise is named after William Healey Dall, who collected the first specimen in waters off Alaska in the late 19th century. Dall’s and all other porpoises belong to a family called Phocoenidae. Like dolphins they belong to the sub-order Odontocete (toothed whales), which in turn belongs to the order Cetacea which includes all whales, dolphins and porpoises. Although dolphins and porpoises look very similar, porpoises have been distinct from their dolphin relatives for around 11 million years.

Dall’s porpoises (Phocoenoides dalli) are the largest of all phocoenids. They have powerful and stocky bodies with adults measuring between 1.8 and 2 metres, weighing in at between 130 to 220 kgs. Dall’s porpoises have a small head and lack a rostrum (beak), although they do have a sloping forehead like dolphins. Their flippers are small, pointed, and located near the head. The dorsal fin is triangular in shape with a hooked tip.

There are two distinct colour types of Dall’s porpoises, distinguished by the size of the white flank patch. In truei-type Dall’s have a white ventral patch which begins at flipper level, while dalli-type Dall’s have a smaller white patch starting at the dorsal fin level. There are occasionally Dall’s porpoises with uniform white or black colour.

Porpoises are generally much smaller and stockier than dolphins and tend to live shorter lives. They lack the prominent beak that dolphins usually have and their dorsal fins are usually triangular shaped compared to the generally curved dorsal fins of dolphins. Porpoises have flat spade-shaped teeth with long, sharp upper edges while dolphins have pointed cone shaped teeth. Another difference is that there are more fused neck vertebrae in porpoises than in dolphins, which limits the range of mobility in the porpoise neck.

Dall's porpoises do not exhibit the typical shy and secretive behaviour typical of most other porpoises. They are very fast swimmers and can reach speeds of 55km/h, making them the fastest swimmers of all small cetaceans, at least for short bursts. They often swim in a zigzag pattern with fast, jerky, steep angled turns. It is this erratic swimming behaviour that makes them so easily identified, especially when they demonstrate their unique rooster-tailing roll – the term used to refer to the cone of water coming off the head of the porpoise (which looks like a rooster’s tail) due to the quick speed and steep angles at which the species surfaces. The rooster tail creates a hollow airspace in the water which allows the porpoise to continue breathing whilst swimming.

Dall's porpoises are usually found in small groups of 2 to 12 individuals, although groups of up to several thousand are not unheard of. The groups appear to be fluid, often forming then breaking up in order to feed and play. Dall’s are known to associate with Pacific white-sided dolphins and have been observed bow riding along with various species of whales.

Dall's porpoises are found throughout the northern Pacific and adjacent seas. At least eight separate stocks have been identified, based on pigmentation and the distribution of mother and calf pairs. Dalli-type porpoises are widely distributed in the North Pacific, the Bering Sea and the Sea of Japan. The truei-type is limited to one stock that winters of the north east coast of Japan and summers in the central Okhotsk sea. Dall’s porpoises are a deep water species, so when they approach the coast they usually follow canyons or deep channels. They are also commonly observed in sounds and inland passages where these meet the open sea.

Very little is known about Dall’s porpoise mating patterns. The calving period is usually in the Spring or Summer, with an 11-12 month gestation period followed by a two year lactation period. Observations of single females during the breeding season indicate that females do not become pregnant every year.

Dall's porpoises are important predators of fish and cephalopods (e.g. octopuses and squids) in the ecosystems they inhabit. They feed primarily at night and appear to be opportunistic feeders with a wide variety of small fish and squid found in their diet. Studies of their diet suggest that Dall's porpoises change prey items and feeding times based on supply. Deepwater species have been identified in their diet implying that feeding has occurred at great depth.

Killer whales and sharks are believed to be the Dall’s porpoise's primary predator, other than man. However, they often escape predation due to their large body size, agility in the water and gregarious nature.

A key threat to Dall’s porpoises is the high concentrations of organochlorines (organic compounds such as DDT and other pesticides) and mercury they accumulate. Studies have suggested that the pollutants could have detrimental effects on reproduction and calf survival. The high level of methyl-mercury in Dall’s porpoise meat has led to the species being specifically included in a 2005 government food safety guideline in Japan, which warns pregnant women to limit their consumption of Dall’s porpoise and other whale and dolphin products.

Dall’s porpoises have historically been subject to very large bycatches in fishing gear, however current levels are not reported and not known. A significant bycatch of Dall’s porpoises likely still occurs in the Japanese salmon drift-net fishery in the Okhotsk Sea. From 1993 to 1999 it was estimated to be between 9,354 and 14,592 Dall’s porpoises, with animals taken from both of the populations that are hunted by Japan.