Gladius (Latin: [ˈɡɫad̪iʊs̠]) is a Latin word meaning "sword" (of any type), but in its narrow sense it refers to the sword of ancient Roman foot soldiers. Early ancient Roman swords were similar to those of the Greeks, called xiphe (plural; singular xiphos). From the 3rd century BC, however, the soldiers of the Roman Republic adopted a sword based on the celtic sword used by the Celtiberians in Hispania late into the Punic Wars, known in Latin as the gladius hispaniensis, meaning "Hispanic-type sword". New variants of the gladius, such as the "Mainz gladius" and the "Pompeii gladius", were used from the first century AD and during the early centuries of the Roman Empire; in the third century AD the Roman infantry replaced the gladius with the "spatha" (which was already used by Roman cavalrymen).

A fully equipped Roman legionary after the reforms of Gaius Marius was armed with a shield (scutum), one or two javelins (pila), a sword (gladius), often a dagger (pugio), and, perhaps in the later empire period, darts (plumbatae). Conventionally, soldiers threw pila to disable the enemy's shields and disrupt enemy formations before engaging in close combat, for which they drew the gladius. A soldier generally led with the shield and thrust with the sword.