Bessus or Bessos (Old Persian: *Bayaçā; Ancient Greek: Βήσσος), also known by his throne name Artaxerxes V (Old Persian: 𐎠𐎼𐎫𐎧𐏁𐏂𐎠 Artaxšaçāʰ; Ancient 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.

"Bessus" (Βήσσος) is the Greek transliteration of the Old Persian name *Bayaçā ("protecting from fear"). Artaxerxes is the Latin form of the Greek Artaxérxes (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 (r. 336–330 BC), 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. According to the Iranologist Richard Foltz, the eastern satrapies were practically Bessus' personal domain. Following the Persian defeat at the Battle of Issus against the Macedonian king Alexander the Great (r. 336–323 BC) in 333 BC, Darius III summoned Bessus to aid him.

Bessus later took part in the Battle of Gaugamela against Alexander in 331 BC, where he supplied Darius III with a contingent composed of Bactrians, Sogdians, Indians, as well as his Saka allies. He led the cavalry on the left wing of the Persian forces, with the intention of crippling Alexander's attack on that flank. Following the Persian defeat at the battle, Bessus followed Darius III in his flight to the city of Ecbatana in Media. There Bessus conspired against Darius III with other Persian grandees, such as the chiliarch Nabarzanes, and Barsaentes, the satrap of Arachosia–Drangiana. Together they arrested Darius III in mid-330 BC, with Bessus being chosen as the leader of the Achaemenid forces, probably due to his Achaemenid descent. The arrest of Darius III gave Alexander the pretext of avenging him. Fleeing from the pursuing Macedonian forces, Bessus and the rebels carried Darius III in a covered wagon, reportedly in golden chains. In order to buy some time for their escape, Bessus and his co-conspirators killed Darius III and left his body by the road. The murder took place near Hecatompylus, in July 330 BC. Darius III's body was later found by Alexander, who had him buried in the royal crypt in Persepolis.

In the autumn of 330, at the Bactrian capital of Bactra, Bessus declared himself King of Kings of the Achaemenid Empire, assuming the royal name of Artaxerxes V. There he attempted to hold the eastern part of the empire against Alexander. His domain encompassed Bactria, the main center; Sogdia, administered by officers such as Spitamenes and Oxyartes; the nomadic Iranian tribes of Central Asia; Aria, governed by the satrap Satibarzanes, who had initially surrendered to Alexander; Arachosia-Drangiana, governed by Barsaentes; Parthia and Hyrcania, governed by Nabarazanes following his appointment by Bessus; and western India. Bessus' usurpation and the invigorated resistance in Central Asia created a new problem for the Macedonians. If they failed to confront Bessus, his usurpation could be seen as change in government, thus rendering Darius III's death irrelevant. However, Bessus' empire quickly started to fall apart; Nabarzanes, accepting his hopeless position, surrendered to Alexander and was pardoned with the help of Bagoas. Satibarzanes was defeated and killed by the Macedonian forces in 329 BC, before Bessus was able to help him. Around the same time, Barsaentes fled to India to escape the Macedonian forces.