The gladius, also known by its full name, the gladius hispaniensis, was the shortsword used by the Roman army through much of its history. The historian Polybius says that the gladius was introduced into the Roman army during the Second Punic War, inspired by the weapons of the Spanish Celts.

There are two known, common patterns of gladius, with a third, transitional type. The oldest of the three is known as the Mainz pattern. The Mainz gladius was 50 to 56 centimeters in blade length and was slightly wasp- waisted. It ran between six and eight centimeters in width, with a long point. The Pompeii pattern is a smaller weapon at about five centimeters in blade width,and ran between 40 and 56cm long with parallel edges and a short point. The transitional, Fulham type has a long point, parallel edges, and slight flaring at the hilt. All gladius blades were double-edged (though primarily thrusting swords, they could cut), diamond-shaped in cross-section, and had no grooves or fullers in the blade.

The gladius was well-suited to the tightly disciplined, close-order methods of fighting used by the Roman legions. The upwards thrusting technique used with the gladius was designed to bypass the ribcage from beneath, striking vital organs, and also managed to keep the legionary behind his large scutum - a major problem with slashing attacks is that they would leave the right side of the body wide open to blows. As long as a legion could maintain order, the gladius would remain a fearsome weapon in battle, even against far larger melee weapons.