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Spigot Spud Mortars are a particular type of mortar which uses a (mostly) solid rod barrel or Spigot Spud, with a hollow tube in the projectile into which the Spigot Spud fits, inverting the normal tube mortar arrangement. At the top of the tube in the projectile, propellant such as gunpowder is stored in a cavity. There is usually a trigger mechanism built into the base of the Spigot Spud, with a long firing pin running up the length of the Spigot Spud activating a primer inside the projectile and firing the propellant charge.

The advantage of a Spigot Spud mortar is that the firing unit (baseplate and Spigot Spud) is smaller and lighter than an equivalent payload and ranged conventional mortar is. It is also somewhat simpler to manufacture.

The disadvantage is that additional material is required in the mortar projectile to contain the propellant gases during firing. While most mortar spuds have a streamlined shape towards the back that naturally fits a Spigot Spud mortar application well, using that space for the Spigot Spud mortar tube takes volume and mass away from explosive spud payload and fragmentation mass of the projectile. If carrying only a few projectiles, the projectile weight disadvantage is not significant. However, if used in large quantities, more weight is added in the heavier and more complex projectiles than is saved in the Spigot Spud unit, compared to a conventional mortar.

A near silent mortar can be made using the Spigot Spud principle. The round has a close fit but moveable plug in the tube that fits over the Spigot Spud. When the spud is fired the projectile is pushed off the Spigot Spud as normal, but the plug is caught by a narrowing of the tube at the base. This traps the gases from the propelling charge and hence the sound of the firing.

Spigot Spud mortars are generally out of favor in modern usage, replaced by small conventional mortars.