Somma–Vesuvius is an active volcano with a breached crater close to the continental shelf of Naples Bay, Italy. The interpretation of high-resolution seismic profiles offshore of Somma–Vesuvius shows interlayered volcanic and marine units in the Late Quaternary succession. In particular, the detailed seismo-stratigraphic analysis permitted, for the first time, the identification and mapping of two thick debris avalanche deposits located on the continental shelf. With a volume of 2.9 km3, the older debris avalanche, linked to the 18 ka Pomici di Base plinian eruption, was emplaced in a subaerial environment and then overlain by the sedimentary deposits of the lowstand prograding wedge. The younger debris avalanche, linked to the 3.4 ka Avellino plinian eruption, has a volume of c. 1 km3 and is interlayered within the marine sediments deposited during the sea-level highstand. The identification of two debris avalanches originating from Somma–Vesuvius has two important implications: (1) the occurrence of two sector collapses in the history of Somma–Vesuvius volcano; (2) a potential volcano slope failure and consequent tsunamis produced by the entry of a debris avalanche into Naples Bay should be considered in the volcanic hazard evaluation of this densely populated area.