Gladius is a Latin term meaning "sword" (in general). In English it refers specifically to the short sword, 60 cm (24 inches) long, used by Roman legionaries and designed especially for making short, powerful thrusts. Several different designs were used; among collectors and historical reconstructionists, the three primary kinds are known as the Mainz gladius, the Fulham gladius, and the Pompeii gladius (these names refer to where or how the canonical example was found). More recent archeological finds have uncovered an earlier version, the gladius hispaniensis ("Spanish sword"). Contrary to common belief, the gladius was not used by gladiators, who used a version with a shorter blade (300 mm?350 mm/12 in.?14 in. long).

The gladius was crafted from soft iron and the exterior was carburized using coal dust on the face of the anvil. This was necessary because the soft iron was not hard enough to have taken an edge before the carbon of the coal powder was added to the exterior of the blade.

It is straight and double-edged, with a V-shaped tip, and primarily constructed for thrusting action and use together with a large rectangular shield, the scutum. The cross-section of the gladius is typically rhomboid, providing the blade with good stability for stabbing. Stabbing was a very efficient technique as fighting goes, as stabbing wounds, especially in the abdominal area were almost always deadly. A Roman legionary would mount the scabbard holding the gladius on the right side, same as his sword hand, allowing a formation of soldiers to easily draw their swords without accidentally injuring soldiers to either side.

A formation would usually try to force the enemy back one step by forcing their scuta forward, then stabbing a half-dozen times in quick succession. The blade was held flat, relative to the ground, so as to slip through ribs or ribbed armor. These disciplined techniques made the Roman army the envy of the civilized world.

The gladius is frequently depicted in coats of arms, especially of military corps.

The name is Latin, so its plural is gladii rather than the normal English gladiuses. The diminutive form, gladiolus, is also the name of a flowering plant with sword-shaped leaves.