Bessus or Bessos (*Bayaçā; Βήσσος), also known by his throne name Artaxerxes V (𐎠𐎼𐎫𐎧𐏁𐏂𐎠 ; Ἀρταξέρξης; died summer 329 BC), was a Persian satrap of the eastern Achaemenid satrapy of Bactria, as well as the self-proclaimed King of Kings of the Achaemenid Empire from 330 to 329 BC.
A member of the ruling Achaemenid dynasty, Bessus came to power shortly after killing the legitimate Achaemenid ruler Darius III (336-330), and subsequently attempted to hold the eastern part of the empire against the Macedonian king Alexander the Great (336-323). His realm quickly started to fall apart, including Bactria, which was the main center. Fleeing into Sogdia, he was arrested by his own officers, who handed him over to Alexander, who had him executed at Ecbatana.
Bessus appears in the 11th-century Persian epic Shahnameh under the name of Janusipar/Janushyar.
"Bessus" (Βήσσος) is the Greek transliteration of the Old Persian name *Bayaçā ("protecting from fear"). Artaxerxes is the Latin form of the Greek Artaxerxes (Αρταξέρξης), itself from the Old Persian Artaxšaçā (𐎠𐎼𐎫𐎧𐏁𐏂𐎠, "whose reign is through truth"). It is known in other languages as; Elamite Ir-tak-ik-ša-iš-ša, Ir-da-ik-ša-iš-ša; Akkadian Ar-ta-ʾ-ḫa-šá-is-su; Middle Persian and New Persian Ardašīr.
Nothing is known about the background of Bessus, except that he belonged to the ruling Achaemenid dynasty. During the reign of the King of Kings Darius III (336-330), Bessus was made the satrap of Bactria, an important satrapy in the eastern part of the empire. Bactria, which had been under Achaemenid rule since 545–540 BC, was prosperous and populous. As shown by archaeological findings, agriculture, trade, commerce and handicraft thrived in the area. The office of satrap of Bactria seems to have occasionally been equivalent to a "vice‐king of the east" type of position.
As the satrap of Bactria, Bessus was able to exert his rule over Sogdia to the north, and regions that bordered India. He managed to retain the loyalty of the Iranian nomadic groups in Central Asia, the Saka, the Dahae and the Massagetae.