Patterns of coloration are highly variable, but Dall's porpoises are mostly black, have white to grey patches on the flank and belly, and frosting on the dorsal fin and trailing edge of the fluke.

[6][7] They are the largest porpoise species, growing up to 2.3 metres (7 ft 7 in) in length and weighing 130–220 kilograms (290–490 lb).

[7] Sexual dimorphism is apparent in body size and shape, with mature males being larger, developing a deeper caudal peduncle, and having a dorsal fin that is significantly angled forward in comparison to a female's.

[2][6][11] Stomach content analyses have also found cases of crustacean consumption, including krill and shrimp, but this is abnormal and likely not an important part of their diet.

[16] They have, however, been observed in association with resident killer whales, engaging in apparent play behaviors with their calves, and swimming with them.

[17] One recognizable Dall's porpoise was observed travelling with the AB pod of resident orca from May through October 1984.

They are commonly seen approaching boats to bowride, and they will also ride on the waves formed at the heads of larger swimming whales.

[27] Smaller numbers, from several hundred to a few thousand, are estimated to have been bycaught in Japanese salmon fisheries in US waters and in the Bering Sea from 1981 to 1987.

[28] Driftnet and trawl fisheries still operate in some areas throughout their range,[2] with particularly high levels of bycatch in Russian waters.

The hunt of Dall's porpoises has been criticized by scientific committees which question the sustainability of large quotas on regional populations.

[33] Environmental contaminants, including dichlorodiphenyldichloroethylene (DDE) and polychlorinated biphenyls (PCBs), are another threat to Dall's porpoises.

Pollutants accumulate in the blubber layer, and in high concentrations can reduce hormone levels, affect the reproductive system,[34] and result in calf death.

[2] Levels of both bycatch and commercial hunting are likely underestimates because they account only for reported data; however, there is no evidence for a range-wide decline of the species.