Middle Persian, also known by its endonym Pārsīk or Pārsīg (Inscriptional Pahlavi script: 𐭯𐭠𐭫𐭮𐭩𐭪, Manichaean script: 𐫛𐫀𐫡𐫘𐫏𐫐‎, Avestan script: 𐬞𐬀𐬭𐬯𐬍𐬐) in its later form, is a Western Middle Iranian language which became the literary language of the Sasanian Empire. For some time after the Sasanian collapse, Middle Persian continued to function as a prestige language. It descended from Old Persian, the language of the Achaemenid Empire and is the linguistic ancestor of Modern Persian, the official language of Iran (also known as Persia), Afghanistan (Dari) and Tajikistan (Tajik).

"Middle Iranian" is the name given to the middle stage of development of the numerous Iranian languages and dialects. The middle stage of the Iranian languages begins around 450 BCE and ends around 650 CE. One of those Middle Iranian languages is Middle Persian, i.e. the middle stage of the language of the Persians, an Iranian people of Persia proper, which lies in the south-western Iran highlands on the border with Babylonia. The Persians called their language Parsig, meaning "Persian".

Another Middle Iranian language was Parthian, i.e. the language of the northwestern Iranian peoples of Parthia proper, which lies along the southern/south-eastern edge of the Caspian sea and is adjacent to the boundary between western and eastern Iranian languages. The Parthians called their language Parthawig, meaning "Parthian". Via regular sound changes Parthawig became Pahlawig, from which the word 'Pahlavi' eventually evolved. The -ig in parsig and parthawig was a regular Middle Iranian appurtenant suffix for "pertaining to". The New Persian equivalent of -ig is -i.

When the Arsacids (who were Parthians) came to power in the 3rd-century BCE, they inherited the use of written Greek (from the successors of Alexander the Great) as the language of government. Under the cultural influence of the Greeks (Hellenization), some Middle Iranian languages, such as Bactrian, also had begun to be written in Greek script. But yet other Middle Iranian languages began to be written in a script derived from Aramaic. This occurred primarily because written Aramaic had previously been the written language of government of the former Achaemenids, and the government scribes had carried that practice all over the empire. This practice had led to others adopting Imperial Aramaic as the language of communications, both between Iranians and non-Iranians. The transition from Imperial Aramaic to Middle Iranian took place very slowly, with a slow increase of more and more Iranian words so that Aramaic with Iranian elements gradually changed into Iranian with Aramaic elements. Under Arsacid hegemony, this Aramaic-derived writing system for Iranian languages came to be associated with the Parthians in particular (it may have originated in the Parthian chancellories), and thus the writing system came to be called pahlavi "Parthian" too.

Aside from Parthian, Aramaic-derived writing was adopted for at least four other Middle Iranian languages, one of which was Middle Persian. In the 3rd-century CE, the Parthian Arsacids were overthrown by the Sassanids, who were natives of the south-west and thus spoke Middle Persian as their native language. Under Sassanid hegemony, the Middle Persian language became a prestige dialect and thus also came to be used by non-Persian Iranians. In the 7th-century, the Sassanids were overthrown by the Arabs. Under Arab influence, Iranian languages began to be written in Arabic script (adapted to Iranian phonology), while Middle Persian began to rapidly evolve into New Persian and the name parsik became Arabicized farsi. Not all Iranians were comfortable with these Arabic-influenced developments, in particular, members of the literate elite, which in Sassanid times consisted primarily of Zoroastrian priests. Those former elites vigorously rejected what they perceived as 'Un-Iranian', and continued to use the "old" language (i.e. Middle Persian) and Aramaic-derived writing system. Numerous examples can be identified through the myriad of Middle Persian Zoroastrian scriptures, such as the Denkard, Shkand-gumãnig Vizār, and many more. In time, the name of the writing system, pahlavi "Parthian", began to be applied to the "old" Middle Persian language as well, thus distinguishing it from the "new" language, farsi. Consequently, 'pahlavi' came to denote the particularly Zoroastrian, exclusively written, late form of Middle Persian. Since almost all surviving Middle Persian literature is in this particular late form of exclusively written Zoroastrian Middle Persian, the term 'Pahlavi' became synonymous with Middle Persian itself.

The ISO 639 language code for Middle Persian is pal, which reflects the post-Sasanian era use of the term Pahlavi to refer to the language and not only the script.

In the classification of the Iranian languages, the Middle Period includes those languages which were common in Iran from the fall of the Achaemenid Empire in the fourth century BCE up to the fall of the Sasanian Empire in the seventh century CE.

The most important and distinct development in the structure of Iranian languages of this period is the transformation from the synthetic form of the Old Period (Old Persian and Avestan) to an analytic form: