Harbour porpoises Phocoena phocoena are common in continental shelf areas of the North Atlantic, but little information is available on their occurrence outside coastal areas. In this study, 30 harbour porpoises were actively caught in West Greenland and instrumented with satellite transmitters to document their seasonal movements and diving behaviour. The porpoises displayed long-range oceanic movements within the North Atlantic, especially during winter/spring where they moved over areas with water depths >2500 m. While offshore, 2 females demonstrated an average maximum dive depth of 248 m, with the deepest dive reaching 410 m. This behaviour is in contrast to 71 porpoises tagged in Danish waters of the North Sea which did not leave the continental shelf but showed a preference for areas with shallow waters year round, even when at the edge of the continental shelf where greater depths were available. Six tags from Greenland transmitted long enough (up to 3 yr) to demonstrate extensive movements and strong site fidelity to the tagging site in West Greenland the following summer. This study documents that harbour porpoises use oceanic habitats and can dive to depths that enable mesopelagic foraging, while repeatedly demonstrating summer site fidelity to coastal areas.