The SPAS-12 was designed during the 1970s as a semi-automatic military or law enforcement shotgun, and production began in October 1979. The SPAS-12 can function on two settings. The first is "automatic" (more accurately, self-loading semi-automatic) - the foregrip is locked forward and a gas system serves to load the next round without further effort from the operator. In automatic mode, the gun can expend around 4 rounds per second, one round per trigger pull. The second setting is pump-action - the foregrip must be manually pumped back and forth after every shot to eject the spent shell and load a new round from the internal magazine. This mode is required to reliably fire low-pressure ammunition such as tear gas rounds or less-lethal bean bags. Switching between firing modes is done by pressing a button under the foregrip, and sliding the foregrip slightly forwards or backwards until it clicks into position.

The SPAS-12 also has a magazine cut-off feature that, when activated, prevents the loading of a new round from the internal magazine when the slide is opened. This allows the operator to load a specialized round into the chamber without going through the entire magazine first. Early SPAS-12 models also featured a lever-type safety (pictured), but it tended to malfunction. This was later replaced by a push-button safety.