Phoca vitulina, commonly known as the harbor seal, is found along the temperate and subarctic coastal waters of the Northern Hemisphere. Its geographical distribution covers both the Atlantic and Pacific Oceans. Specific regions include the coasts of North America, stretching from Alaska down to Baja California in the Pacific, and from the northeastern United States to maritime Canada and Greenland in the Atlantic. In Europe, they are found along the coasts of Ireland, the United Kingdom, Scandinavia, and as far south as France and Spain.

Harbor seals typically inhabit coastal waters and estuaries, preferring sheltered areas such as bays, inlets, and harbors. They are commonly found on sandy, rocky, or muddy substrates and need access to haul-out sites where they can rest and breed. These sites might be on beaches, rocky outcrops, or ice floes. Environmental conditions include temperate to cold climates with varying seasonal temperatures.

As semi-aquatic pinnipeds, harbor seals play an essential role in their ecosystem as both predators and prey. They primarily feed on fish, squid, and crustaceans, showing opportunistic and generalist feeding habits. Consequently, they contribute to the control of fish populations. Harbor seals are also preyed upon by Orcas and large sharks, which positions them as integral components within the marine food web.