Immortals or Persian Immortals was the name given by Herodotus to an elite heavy infantry unit of 10,000 soldiers in the army of the Achaemenid Empire. The unit served in a dual capacity through its role as imperial guard alongside its contribution to the ranks of the Persian Empire's standing army. While it primarily consisted of Persians, the Immortals force also included Medes and Elamites. Essential questions regarding the historic unit remain unanswered because authoritative sources are missing.

Heavy infantry consisted of heavily armed and armoured infantrymen who were trained to mount frontal assaults and/or anchor the defensive center of a battle line. This differentiated them from light infantry who are relatively mobile and lightly armoured skirmisher troops intended for screening, scouting, and other tactical roles unsuited to soldiers carrying heavier loads. Heavy infantry typically made use of dense battlefield formations, such as shield wall or phalanx, multiplying their effective weight of arms with force concentration.