The SPAS 12 was designed to function primarily as a semi-automatic firearm, with the pump-action mode used 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 has a magazine cut-off feature that can prevent the loading of a new round from the internal magazine when the gun is cycled. This allows the operator to load a specialized round into the chamber without going through the entire magazine first. Another unique feature of the SPAS-12 was the hook seen on folding stock variants. This hook could be rotated in 90 degree increments so that it would fit under the user's forearm when the stock was extended. With the stock supported under the forearm the gun could theoretically be fired with one hand, allowing the user to fire around cover or use their support hand for other tasks.

Early SPAS-12 models featured a lever-type safety, but over time it would begin discharging the firearm when switched on or off. This was eventually recalled by Franchi and replaced by a push-button crossbolt safety. Many guns remain with the lever-type safety and are unsafe to use.

The barrel of the SPAS 12 was externally threaded to accept a wide variety of attachments, from chokes to gas grenade launchers. One interesting and particularly rare attachment, called a "diverter", spread shot vertically or horizontally. All barrel attachments are considered rare, and demand premium pricing on the secondhand market.

The first and most common variant of the SPAS-12 came with a metal folding stock and had a magazine capacity of eight rounds. Early models could be had with a detachable wooden stock, though this is rarely seen. After the United States imposed import restrictions on the type in 1989, a version was released with a synthetic fixed stock and five or six round capacity to comply with regulations. Various barrel lengths were seen on the SPAS-12, ranging from a 18" "shorty" to a 24" UK legal barrel length. The most common barrel lengths encountered are 21" and the slightly less common 19.5".

Franchi released two other shotguns based on the SPAS-12 platform; the LAW-12 and the SAS-12. The LAW-12 was semi-automatic only while the SAS-12 was pump-action only. These two "sister" shotguns accepted all SPAS-12 accessories and could share many other components, notably the trigger packs and stocks. The SAS was unusual in that it could accept 3" shells, whilst the SPAS and LAW could only accept 2 3/4" shells.

The Franchi SPAS-15 is the successor to the SPAS-12. It is also a semi-automatic/pump shotgun, but uses box magazines instead of the internal tube magazine of the SPAS-12.