The Old Persian language, one of the two directly attested Old Iranian languages (the other being Avestan, the language of Zoroastrian religious tradition), appears primarily in the inscriptions, clay tablets, and seals of the Achaemenid era (about 600 BCE to 300 BCE).

Examples of Old Persian have been found in what is now present-day Iran, Iraq, Turkey and Egypt, the most important attestation by far being the contents of the Behistun Inscription (dated to 525 BCE).

Recent research into the vast Persepolis Fortification Archive at the Oriental Institute at the University of Chicago have unearthed Old Persian tablets (2007).

This new text shows that the Old Persian language was a written language in use for practical recording and not only for royal display.

By the fourth century, the late Achaemenid period, the inscriptions of Artaxerxes II and Artaxerxes III differ enough from the language of Darius' inscriptions to be called a "pre-Middle Persian," or "post-Old Persian."

Old Persian now begins to evolve into Middle Persian, which is in turn the genetic ancestor of New Persian.