New wide-angle seismic data were gathered along a 230 km long profile that runs east–west across a deep structural feature in the Porcupine Basin, offshore Ireland, known as the Porcupine Arch. Ocean bottom seismometers were deployed at 3–4 km intervals and seismic sources fired every 120 m along it. Prominent primary and secondary arrivals indicate that the continental crust is extremely thin (locally less than 2 km) across the basin centre. The sedimentary succession is up to 12 km thick and comprises three distinctive seismic layers. The two uppermost layers are interpreted as mostly a post-rift succession of Cretaceous and Cenozoic strata. The lowest layer thins rapidly towards the basin centre and is interpreted as a succession of predominantly Jurassic synrift sediments. A strong asymmetry in both the geometry of the crust and the sedimentary layers is probably related to a simple shear mode of extension and the subsidence that it induced. Crustal thinning is far greater than in the adjacent Rockall Basin and local exhumation of continental mantle lithosphere may have occurred in parts of the Porcupine Basin. Low Pn velocities beneath the Porcupine Arch are compatible with larger amounts of mantle serpentinization than in the Rockall Basin.