The Parni (/ˈpɑːrnaɪ/; Ancient Greek: Πάρνοι, Parnoi) or Aparni (/əˈpɑːrnaɪ/; Ἄπαρνοι, Aparnoi) were an East Iranian people who lived around Ochus (Ancient Greek: Ὧχος Okhos) (Tejen) River, southeast of the Caspian Sea but it is believed that their original homeland may have been southern Russia from where they emigrated with other Scythian tribes. The Parni were one of the three tribes of the Dahae confederacy.
There is no unambiguous evidence of the Parni in native Iranian language sources,cf.  and all references to these people comes from Greek and Latin accounts. In these accounts, which are not necessarily contemporaneous, it is difficult to unambiguously identify references to the Parni due to inconsistency of Greek/Latin naming and transliteration, and/or the similarity to names of other tribes such as the Sparni or Apartani and the Eparnoi or Asparioi. It may also be that the Parni are related to one or more of these other tribes, and that "their original homeland may have been southern Russia from where they emigrated with other Scythian tribes."
The location of the Parni Dahae immediately south-east of the Caspian Sea was derived from by Strabo's Geographica (Book 11, 1st century BCE). The ethnonym of the Dahae was the root of the later placename Dahestan or Dihistan – a region straddling the present regions of Turkmenistan and Iran. So little is known of the Dahae, including the Parni, that – in the words of A. D. H. Bivar – even the location and name of their capital city "if indeed they possessed one" is now unknown."  "an urban center of the ancient Dahae (if indeed they possessed one) is quite unknown."  A later archaeological site in the region, known as Dehistan/Mishrian, is located in the Balkan Region of Turkmenistan.
The language[c] of the Parni is not directly attested but is assumed to be one of the eastern substrates of the subsequently recorded Parthian language, which the Parni eventually adopted. To the "incoming Parni may be ascribed a form of speech showing a strong east Iranian element, resulting from their proximity on the steppe to east Iranian Sakas." Through the influence of the Parthians in Armenia, traces of the Parni language survive as "loan-words in Armenian."
The language of the Parni "was described by Justin as 'midway between Scythian and Median [and] contained features of both'" (41.1.10). Justin's late (3rd century) opinion is "no doubt slightly exaggerated," and is in any case of questionable veracity given the ambiguity of names.
In 247 BCE, Andragoras, the Seleucid governor (satrap) of Parthia ("roughly western Khurasan") proclaimed independence from the Seleucids, when—following the death of Antiochus II—Ptolemy III seized control of the Seleucid capital at Antioch, and "so left the future of the Seleucid dynasty for a moment in question."
Meanwhile, "a man called Arsaces, of Scythian or Bactrian[a] origin, [was] elected leader of the Parni tribes." Following the secession of Parthia from the Seleucid Empire and the resultant loss of Seleucid military support, Andragoras had difficulty in maintaining his borders, and about 238 BCE—under the command of "Arsaces and his brother Tiridates"—the Parni invaded Parthia and seized control of Astabene (Astawa), the northern region of that territory, the administrative capital of which was Kabuchan (Kuchan in the vulgate).
A short while later, the Parni seized the rest of Parthia from Andragoras, killing him in the process. Although an initial punitive expedition by the Seleucids under Seleucus II was not successful, the Seleucids under Antiochus III recaptured Arsacid controlled territory in 209 BCE from Arsaces' (or Tiridates'[b]) successor, Arsaces II. Arsaces II sued for peace and accepted vassal status, and it was not until Arsaces II's grandson (or grand-nephew) Phraates I, that the Arsacids/Parni would again begin to assert their independence.
For the historiographers upon whose documentation the reconstruction of early Arsacid history depends, the Parni had by then become indistinguishable from the Parthians.
The seizure of Astabene in 238 BCE nominally marks the beginning of the Arsacid era, which is named after Arsaces, and the name adopted by all Parthian kings. "Arsaces" is a variant of (also Greek) "Artaxerxes," and the Arsacid dynasts laid claim to descend from Artaxerxes II. Beginning from Astabene and Parthia (which would subsequently be extended southwards to include much of present-day Sistan), the Arsacids gradually subjugated many of the neighboring kingdoms, most of which were thereafter controlled as vassalries. Beginning with the successful revolt - in 224 CE - of an erstwhile vassal of Stakhr named Ardashir (in Greek again "Arsaces"/"Artaxerxes"), the Arsacid/Parthian hegemony began to yield to a Sassanid/Persian one.
The name "Parni" reappears in Sassanid-era documents to identify one of the seven Parthian feudal families allied with the Sassanid court. However, this family is not attested from Arsacid times, and the claim to the "Parni" name is (like four of the six other families) "in all probability not in accordance with reality." "It may be that [...] members of them made up their own genealogies in order to emphasize the antiquity of their families."
It has been suggested that the Parnau Hills (Paran Koh) bear the name of the Parni.