Arsaces I of Parthia (B. ??? BC; R. ca. 250 BC - 246 or 211 BC; D. 246 or 211 BC) was the leader of the Parni, who became the King of Parthia and founded the Arsacid Dynasty. A later tradition (preserved by Arrian) claims that Arsaces I and his brother Tiridates I of Parthia were both descendants of the Achaemenid king Artaxerxes II, but there is no historical evidence to support this claim.
Arsaces, seeking refuge before the Bactrian king Diodotus I, invaded Parthia, then a province of the Seleucid Empire, in about 250 BC. According to Arrian he was then killed and was succeeded by his brother. But modern historians believe that he ruled Parthia until 211 BC, when he was succeeded by his son Arsaces II.
After him all the other Parthian kings of the Arsacid Dynasty, amounting to the number of about thirty, officially wear only the name Arsaces. Arsaces is also the person from whom a celebrated descent from antiquity begins.
The name Arsaces in Parthian is spelled 'ršk (Aršak). In Greek it is written Αρσακης. With very few exceptions only the name Αρσακης occurs on the coins of the Parthian kings (in its genitive form ΑΡΣΑΚΟΥ i.e., "[coin] of Arsaces" — together with various epithets), and the obverse generally shows the seated figure of the founder of the dynasty, holding in his hand a strung bow. The Parthian Empire was finally overthrown in AD 226 by Ardashir I (Ardaxšēr), the founder of the Sassanid Dynasty.
In ancient Chinese the name for Parthia was "Anxi" (Ch:安息, read ansik in Middle Chinese, from Old Chinese *Arsǝk ~ *Ansek), a transcription of the dynastic name Arsaces. Anxi was described by the Chinese envoy Zhang Qian, who visited the neighbouring countries of Bactria and Sogdiana in 126 BC and wrote the first known Chinese report on Parthia.
Arsaces issued coins from silver drachms to copper dichakloi. All issues bear some similarity in style to the Seleucid pieces of the same time, although the Parthian Great King's headdress is notably different. The commonest inscription is ΑΡΣΑΚΟΥ ΑΥΤΟΚΡΑΤΟΡΟΣ, translating as Arsaces the Autocrat, however there are many variations on this.