Avestan () is an umbrella term for two Old Iranian languages: Old Avestan (c. 1500 – c. 1000 BCE) and Younger Avestan (c. 1000 – c. 500 BCE). They are known only from their conjoined use as the scriptural language of Zoroastrianism, and the Avesta likewise serves as their namesake. Both are early Eastern Iranian languages within the Indo-Iranian language branch of the Indo-European language family. Its immediate ancestor was the Proto-Iranian language, a sister language to the Proto-Indo-Aryan language, with both having developed from the earlier Proto-Indo-Iranian language; as such, Old Avestan is quite close in both grammar and lexicon to Vedic Sanskrit, the oldest preserved Indo-Aryan language.

The Avestan text corpus was composed in the ancient Iranian satrapies of Arachosia, Aria, Bactria, and Margiana, corresponding to the entirety of present-day Afghanistan as well as parts of Tajikistan, Turkmenistan, and Uzbekistan. The Yaz culture of Bactria–Margiana has been regarded as a likely archaeological reflection of the early "Eastern Iranian" culture that is described in the Zoroastrian Avesta.