Northwest Atlantic harbour porpoises are spread out over the northern hemisphere's continental shelves (the shallower coastal areas of the ocean, generally within 250 kilometres of shore). But as their name implies, they are often sighted close to shore, seeking out harbours and bays -- especially in summer months. In Canada, the northwest Atlantic harbour porpoise ranges from the Bay of Fundy north to northern Labrador, with three distinct populations in Newfoundland-Labrador, the Gulf of St. Lawrence and the Bay of Fundy/Gulf of Maine. These groups regularly travel down to American waters and back to Canada.

The harbour porpoise has a noticeably rounded head that lacks an obvious beak or snout. It has a small, triangular dorsal fin that sits approximately in the middle of its back. Their sides are mottled grayish-white, which becomes almost fully white on their bellies. It looks like they wear a black 'cape' over their backs and sides, but the look of the cape varies quite a bit among individuals. Northwest Atlantic harbour porpoises also may have dark patches on their faces. Males and females usually look the same in terms of colour, but young porpoises are typically darker. Harbour porpoises average just 1.6 metres in length and weigh about 50 kg. Females tend to be larger than the males, and both genders live relatively short lives: few harbour porpoises live to 20 years.