The South Pacific Gyre is the world’s largest expanse of oligotrophic ocean and supports communities of endemic gadfly petrels Pterodroma spp., yet little is known about their foraging ecology in this nutrient-poor environment. We tracked Murphy’s petrels Pterodroma ultima with geolocators from Henderson Island, Pitcairn Islands, for 2 consecutive years (2011 to 2013). During pre-laying exodus, petrels travelled south and southwest of the colony, with males travelling further than females to more productive waters. During incubation, birds foraged at the southern and eastern edges of the Gyre, with some travelling over 4800 km from the colony, the greatest recorded foraging range of any breeding seabird. During non-breeding, the petrels migrated to the Subarctic Gyre in the North Pacific to forage in cool, mesotrophic waters. Habitat models revealed that these birds do not have clear preferences for oceanographic (such as fronts or eddies) or topographic (seamounts) features, generally favouring deep and unproductive waters. Analyses of activity patterns indicated Murphy’s petrels are amongst the most active of all seabirds, particularly during incubation when they spent ca. 95% of their time at sea in flight. The birds did not appear to forage during darkness, but flight activity peaked at dawn, particularly during non-breeding, suggesting they feed on mesopelagic prey that are diel vertical migrants. At-sea protection for such a wide-ranging species would require management at huge spatial scales, and hence in the short term, the principal focus for conservation should be on eliminating the immediate threat from invasive mammals at breeding sites.