A shotgun cartridge, shotshell, or shell is a type of rimmed, cylindrical (straight-walled) ammunition used specifically in shotguns. It is typically loaded with numerous small, spherical sub-projectiles called shot. Shotguns typically use a smoothbore barrel with a tapered constriction at the muzzle to regulate the extent of scattering.

Some cartridges contain a single solid projectile known as a slug (sometimes fired through a rifled slug barrel). The casing usually consists of a paper or plastic tube with a metallic base containing the primer. The shot charge is typically contained by wadding inside the case. The caliber of the cartridge is known as its gauge.

The projectiles are traditionally made of lead, but other metals like steel, tungsten and bismuth are also used due to restrictions on lead, or for performance reasons such as achieving higher shot velocities by reducing the mass of the shot charge. Other unusual projectiles such as saboted flechettes, rubber balls, rock salt and magnesium shards also exist. Cartridges can also be made with specialty non-lethal projectiles such as rubber and bean bag rounds.

Shotguns have an effective range of about 35 m (38 yd) with buckshot, 45 m (49 yd) with birdshot, 100 m (110 yd) with slugs, and well over 150 m (160 yd) with saboted slugs in rifled barrels.

Most shotgun cartridges are designed to be fired from a smoothbore barrel, as "shot" would be spread too wide by rifling. A rifled barrel will increase the accuracy of sabot slugs, but makes it unsuitable for firing shot, as it imparts a spin to the shot cup, causing the shot cluster to disperse. A rifled slug uses rifling on the slug itself so it can be used in a smoothbore shotgun.

Early shotgun cartridges used brass cases, not unlike pistol and rifle cartridge cases of the same era. These brass shotgun hulls or cases resembled large rifle cartridges, in terms of both the head and primer portions of the cartridge, as well as in their dimensions. Card wads, made of felt, leather, and cork, as well as paperboard, were all used at various times. Waterglass (sodium silicate) was commonly used to cement the top overshot wad into these brass casings. No roll crimp or fold crimp was used on these early brass cases, though roll crimps were eventually used by some manufacturers to hold the overshot wad in place securely. The primers on these early shotgun cartridges were identical to pistol primers of the same diameter.

Starting in the late 1870s, paper hulls began replacing brass hulls. Paper hulls remained popular for nearly a century, until the early 1960s. These shotgun cartridges using paper hulls were nearly always roll crimped, although fold crimping also eventually became popular. The primers on these paper hull cartridges also changed from the pistol primers used on the early brass shotgun shells to a primer containing both the priming charge and an anvil, making the shotgun primer taller. Card wads, made of felt and cork, as well as paperboard, were all used at various times, gradually giving way to plastic over powder wads, with card wads, and, eventually, to all plastic wads. Starting from the early 1960s to the late 1970s, plastic hulls started replacing paper hulls for the majority of cartridges and by the early 1980s, plastic hulls had become universally adopted.

Modern shotgun cartridges typically consist of a plastic hull, with the base covered in a thin brass or plated steel covering. Paper cartridges used to be common and are still made, as are solid brass shells. Some companies have produced what appear to be all-plastic shells, although in these there is a small metal ring cast into the rim of the cartridge to provide strength. More powerful loads may use "high brass" shells, with the brass extended up further along the sides of the cartridge, while light loads will use "low brass" shells. The brass does not provide a significant amount of strength, but the difference in appearance gives shooters a way to quickly differentiate between high and low powered ammunition.