Darius II (Old Persian: 𐎭𐎠𐎼𐎹𐎺𐎢𐏁 Dārayavaʰuš; Greek: Δαρεῖος Dareios), also known by his given name Ochus (Ὦχος Ochos), was King of Kings of the Achaemenid Empire from 423 BC to 405 or 404 BC.

Artaxerxes I, who died in 424 BC, was followed by his son Xerxes II. After a month and half Xerxes II was murdered by his brother Sogdianus. His illegitimate brother, Ochus, satrap of Hyrcania, rebelled against Sogdianus, and after a short fight killed him, and suppressed by treachery the attempt of his own brother Arsites to imitate his example. Ochus adopted the name Darius (Greek sources often call him Darius Nothos, "Bastard"). Neither the names Xerxes II nor Sogdianus occur in the dates of the numerous Babylonian tablets from Nippur; here effectively the reign of Darius II follows immediately after that of Artaxerxes I.

Historians know little about Darius II's reign. A rebellion by the Medes in 409 BC is mentioned by Xenophon. It does seem that Darius II was quite dependent on his wife Parysatis. In excerpts from Ctesias some harem intrigues are recorded, in which he played a disreputable part. The Elephantine papyri mention Darius II as a contemporary of the high priest Johanan of Ezra 10:6.