Greek fire was an incendiary weapon used by the Eastern Roman (Byzantine) Empire. The Byzantines typically used it in naval battles to great effect as it could continue burning while floating on water. It provided a technological advantage, and was responsible for many key Byzantine military victories, most notably the salvation of Constantinople from two Arab sieges, thus securing the Empire's survival.

The impression made by Greek fire on the west European Crusaders was such that the name was applied to any sort of incendiary weapon, including those used by Arabs, the Chinese, and the Mongols. These, however, were different mixtures and not the Byzantine formula, which was a closely guarded state secret, a secret that has since been lost. The composition of Greek fire remains a matter of speculation and debate, with proposals including combinations of pine resin, naphtha, quicklime, sulfur, or niter. Byzantine use of incendiary mixtures was distinguished by the use of pressurized nozzles or siphōn to project the liquid onto the enemy.

Although the term "Greek fire" has been general in English and most other languages since the Crusades, in the original Byzantine sources it is called by a variety of names, such as "sea fire" (Ancient Greek: πῦρ θαλάσσιον pyr thalássion), "Roman fire" (πῦρ ῥωμαϊκόν pyr rhomaïkón), "war fire" (πολεμικὸν πῦρ polemikòn pyr), "liquid fire" (ὑγρὸν πῦρ hygron pyr), or "manufactured fire" (πῦρ σκευαστόν pyr skeuastón).