Helicobacter pylori infection is the main cause of gastric cancer. Vacuolating cytotoxin A (VacA) is a H. pylori pore-forming toxin and a key determinant of gastric cancer risk. VacA is secreted as an 88-kDa polypeptide (p88) that upon interaction with host cells induces cytotoxic effects, including cell vacuolation and mitochondrial dysfunction. These effects are currently believed to be due to VacA p88 accumulating inside host cells and forming oligomeric anion-specific channels in membranes of intracellular compartments. However, the molecular nature of intracellular VacA channels in host cells remains undefined. Here we show that VacA within endosomes is rapidly processed into smaller p31/p28 and p37 products that coincide with vacuolating activities. VacA processing requires endosomal acidification and concerted cleavage by multiple endo-lysosomal proteases including cathepsins. In situ structural mapping reveals that upon processing, the toxin’s central hydrophilic linker and globular C-terminus are excised, whereas oligomerization determinants are retained. Congruently, the processed products are constituents of a high-molecular-weight complex inside the host cell ─ which we propose is the intracellular, mature and active VacA pore. These findings suggest that VacA exploits human endosomal machinery for proteolytic processing and intracellular activation.