Arsaces I (/ˈɑrsəsiːz/; Ancient Greek: Ἀρσάκης, Persian: ارشک‎ Aršak) was the founder of the Arsacid dynasty, and after whom all 30+ monarchs of the Arsacid empire officially named themselves. A celebrated descent from antiquity (the Bagratid "line") begins with Arsaces.Arsaces or Ashk has also given name to the city of Ashkabad.

Information on Arsaces I comes solely from non-contemporaneous (1st century) Greek and Latin reiterations of Arsacid legends (Arrian i, preserved in Photius and Syncellus, and Strabo xi). The dates of Arsaces' birth and death are unknown, as is his real name.

In contrast, the circumstances of Arsaces' ascent to power are relatively well known. Around 250 BC, Andragoras, the governor of the Seleucid province of Parthia, proclaimed his independence from the Seleucid monarchs, and made his governorate an independent kingdom. At about the same time, Arsaces – who was of Scythian or Bactrian origin (and may even have been a Bactrian prince) – was elected leader of the Parni, an eastern Iranian tribe. With the Parni, Arsaces seized Astauene (or Astabene), i.e., northern Parthia. Andragoras was killed during his attempts to recover it, which left the Parni in control over the rest of Parthia as well. A recovery expedition by the Seleucids under Seleucus II did not succeed, and Arsaces and the Parni succeeded in holding Parthia proper during Arsaces' lifetime. (Arsaces II lost it in 209 BC to Antiochus III, to whom the Arsacids became vassals for the next 25 years).

Parthia is a region of north-eastern Iran, best known for having been the political and cultural base of the Arsacid dynasty, rulers of the Parthian Empire.

The name "Parthia" is a continuation from Latin Parthia, from Old Persian Parthava, which was the Parthian language self-designator signifying "of the Parthians" who were an Iranian people.

Parthia roughly corresponds to the western half of Khorasan region in northeastern Iran. It was bordered by the Kopet Dag mountain range in the north and the Dasht-e-Kavir desert in the south. It bordered Media on the west, Hyrcania on the north west, Margiana on the north east, and Aria on the south east.

During Arsacid times, Parthia was united with Hyrcania as one administrative unit, and that region is therefore often (subject to context) considered a part of Parthia proper.

As the region inhabited by Parthians, Parthia first appears as a political entity in Achaemenid lists of governorates ("satrapies") under their dominion. Prior to this, the people of the region seem to have been subjects of the Medes, and 7th century BC Assyrian texts mention a country named Partakka or Partukka (though this "need not have coincided topographically with the later Parthia").