Bessus or Bessos (Old Persian: *Bayaçā; Greek: Βήσσος), also known by his throne name Artaxerxes V (Old Persian: 𐎠𐎼𐎫𐎧𐏁𐏂𐎠 Artaxšaçāʰ; Greek: 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 (r. 336–330 BC), and subsequently attempted to hold the eastern part of the empire against the Macedonian king Alexander the Great (r. 336–323 BC). 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.