The underexplored Barents shelf petroleum province is a globally unique example where naturally occurring underpressure is observed offshore and onshore. In the offshore parts of the northern Barents shelf, minor underpressure (up to 23 bar below hydrostatic pressure) is observed in the fault-bounded Mesozoic reservoirs of the Fingerdjupet subbasin. More severe (50 bar below hydrostatic pressure although irregular, occurrences of underpressure are encountered in the Triassic intervals of the neighboring Greater Hoop area. The abnormal pressures extend to the onshore archipelago of Svalbard, where pressures exceeding 60 bar below hydrostatic were encountered during drilling for a CO2 sequestration project. In Svalbard, reservoir pressures were constantly monitored over 3 yr, providing an insight into the reservoir behavior at unique time scales. The low-permeability (<2 md) reservoir in Svalbard is exposed some 15 km to the north of the drill site. Quantitative analysis with the apparent lack of a regional lateral seal suggests a geologically recent origin of underpressure. Evidence that the underpressure extends into the top seal provides further indication to the likely cause of underpressure. Similar to many global occurrences of underpressure in petroleum provinces, the Barents shelf has undergone severe uplift, most recently caused by deglaciation. Well data, outcrop observations, and isotope data combined with the area's geological history indicate that glacial loading, unloading, and erosion, potentially with the aid of natural fractures, is the likely dominant underpressure-generating mechanism.