The weapon is based on the SPAS-12, and has similar pump-action/semi-automatic firing modes. In semi-automatic mode a gas piston drives a bolt carrier and rotating bolt. In pump-action mode the same components are driven by sliding the fore-end backwards. Pump-action mode is required to reliably fire low-pressure (less lethal) ammunition such as tear gas rounds or less-lethal bean bags. Switching between firing modes is done by pressing a button above the foregrip, and sliding the foregrip slightly forwards or backwards. The barrel is chrome lined and features screw-in choke tubes.
Unlike its predecessor, the SPAS-15 is fed by a detachable box magazine. The gun features a folding stock and a pistol grip safety. Amongst Italian troops the weapon is known by the nickname "La Chiave dell'Incursore" (the key of the commando) because it is used to blast the locks of closed doors.