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The harbour porpoise is a chunky animal, with a blunt shortbeaked head. Placed about midway along the back is a short, wide-based, triangular dorsal fin, with small bumps on the leading edge. The flippers are small and somewhat rounded at the tips. The flukes have a concave trailing edge, divided by a prominent median notch; the tips are rounded. The straight mouthline slopes upward towards the eye. Countershading is apparent in the harbour porpoise's colour pattern; the animals are generally dark grey on the back and white on the belly. The sides are intermediate, with the border area often splotched with shades of grey. The flippers and lips are dark; there is a thin, dark grey gape-to-flipper stripe. Nineteen to 28 small, spatulate, blunt teeth line each tooth row. Can be confused with: Harbour porpoises, if seen clearly, should not be confused with any of the various species of dolphins that share their range. The other porpoise that overlaps in the North Pacific, Dall's porpoise, can be confused with this species when backlit fins are seen at a distance. However, the black and white colour pattern and slight difference in dorsal-fin shape of Dall's porpoise will be distinguishable, when seen well.

Most adult harbour porpoises are less than 1.8 m long; maximum length is about 2 m. Females are slightly larger than males. Weights range from 45 to 70 kg for adults. Newborns are 70 to 90 cm long.

Most harbour porpoise groups are small, consisting of less than 8 individuals. They do, at times, aggregate into large, loose groups of 50 to several hundred animals, mostly for feeding or migration. Behaviour tends to be inconspicuous, compared to most dolphins, and harbour porpoises rarely approach boats to ride bow waves. When moving fast, they surface in a behaviour often called pop-splashing. Breaches and other leaps are rarely seen. Harbour porpoises sometimes lie at the surface for brief periods between submergences, although we do not know why they do this. Reproductive biology has been well-studied in some parts of the world. Most calves are born from spring through mid-summer. Harbour porpoises eat a wide variety of fish and cephalopods, and the main prey items appear to vary regionally. Small, non-spiny schooling fish (such as herring and mackerel) are the most common prey in many areas, and many prey species are benthic or demersal.

A major human threat to harbour porpoises throughout their range is incidental capture in fisheries. Many thousands are taken each year in gillnets and in certain areas, incidental catches in herring weirs, cod and salmon traps, purse seines, trawl nets, and longlines also occur. Directed fisheries have occurred in Puget Sound, the Bay of Fundy, Gulf of St. Lawrence, Labrador, Newfoundland, Greenland, Iceland, Black Sea, and the Baltic Sea. Many of these fisheries are now closed, but hunting of harbour porpoises still occurs in a few areas. Greenland and the Black Sea are the only areas where large direct catches have been reported recently. Levels of pollutants in harbour porpoise tissues have been found to be high wherever studied, probably due to the species' coastal nature. Environmental contamination has been implicated, in part, for declines in harbour porpoise populations in Europe and some parts of North America. IUCN: Insufficiently known.