Old Persian was the Iranian language spoken by the ruling class of the Achaemenid Empire, probably reflecting the Southwest Iranian dialect of Persis (see Persia). It is preserved in documents in a cuneiform script superficially modeled on Mesopotamian (Sumero-Akkadian) writing and first used under Darius I in the late 6th century bce. As a spoken language, Old Persian was the direct ancestor of Middle Persian and Modern Persian (Farsi). The script was the first cuneiform writing to be deciphered by modern scholars, starting in 1802 with the pioneering work of Georg Grotefend; this laid the basis for the subsequent decipherment of Mesopotamian cuneiform and the languages written in it, one of the most far-reaching achievements of 19th-century science (see cuneiform).

Of the two Old Iranian languages that survive in written records (the other being Avestan, the language of the Zoroastrian liturgical texts), only Old Persian is attested to in original documents contemporary with when it was spoken. Most are monumental royal inscriptions, often trilingual (Old Persian, Elamite, Babylonian) in the early period, and have been found primarily in the historical regions of Persis, Elam, and Media. Many of these, most famously the massive trilingual inscribed on a high rock face at Bisotun (Behistun) that records the deeds of Darius I, are of immense value to historians. Though there is evidence of the language throughout the reign of Artaxerxes III (d. 338 bce), most of its remains stem from the time of Darius I and Xerxes I; later inscriptions are of lesser quality, devolving into clichéed bricolage and sometimes exhibiting grammatical errors. A smaller number of inscriptions are found on vases and other objects, as well as on seals.