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

Following the death of Artaxerxes I, in 424 BC or 423 BC, there was a struggle for power between his sons. The victor, Ochus, adopted the name Darius (Greek sources often call him Darius Nothos, "Bastard"), in reference to his unattested lineage. His reign was marked by a series of revolts by various satraps and involvement in the Greek Peloponnesian War.

It seems 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 6:10. Darius II is potentially mentioned in the books of Haggai, Zechariah, and Ezra–Nehemiah of the Hebrew Bible (the Christian Old Testament). There is some debate on whether these books refer to Darius the Great though.

Texts from the Babylonian Murashu Archive date the transition from Artaxerxes I to Darius II between December 424 BC and February 423 BC. These Babylonian records do not reference any other contenders for the Persian throne directly, but Classical Greek and Latin historians, primarily Ctesias of Cnidus, describe a struggle for power within the Achamenid royal family. Darius II received his royal name and recognition from supporters in Babylon, while his half-brothers, Xerxes II and Sogdianus, claimed authority elsewhere in the Empire. After just 45 days, Sogdianus orchestrated Xerxes's assassination.

Darius quickly gathered support from influential figures in the Persian Empire, including Sogdianus’s former cavalry commander, Arbarius, Satrap Arsames of Egypt, the influential Paphlagonian eunuch Artoxares, and possibly Satrap Hydarnes of Armenia. Rather than fighting Sogdianus, Darius and his wife and half-sister, Parysatis, arranged to negotiate. When Sogdianus arrived for negotiations, he was seized and executed in a pit of hot ashes.

Soon after defeating his half-brothers, Darius’s full brother, Arsites, rebelled for unclear reasons. Arsites was Satrap of Syria and had the support of Artyphius, son of the earlier rebel Satrap Megabyzus. Darius sent an army to confront his brother under the command of Artasyrus, a Bactrian nobleman. The rebels defeated Artasyrus' army in two engagements, but they were defeated in a third. After that defeat, Artasyrus bribed the Greek mercenaries in Arsites's army to surrender and captured Artyphius. Artyphius was temporarily spared on Parysatis’ advice to demonstrate the new regime’s leniency. When Arsites surrendered, he and Artyphius were both executed with hot ashes as well.

Early in Darius’s reign, Pissuthnes, the Satrap of Lydia, also staged a revolt for unknown reasons. Pissuthnes employed the Athenian general Lycon to command Greek mercenaries on his behalf while Darius sent Tissaphernes, a younger son of Hydarnes, to combat the rebellion. Tissaphernes immediately sent a letter to Lycon, offering the Greek mercenaries greater rewards if they turned on Pissouthnes and joined Tissaphernes’ army. The rebellious satrap was forced to negotiate and surrender. Tissaphernes sent him back to Persia to face execution and became the new Satrap of Lydia as a reward.

Pissuthnes' son, Amorges, continued the rebellion with a band of mercenaries funded by Athens in Iasus. Darius may have been hesitant to pursue Amorges in order to avoid conflict with Athens, but after the disastrous Athenian expedition to Sicily, the king ordered Tissaphernes and the neighboring satrap, Pharnabazus II, to defeat the rebels and exact tribute from Athenian-held territory in Asia Minor. Tissaphernes arranged an alliance with Sparta, and the Spartan navy assisted the Persians in defeating Amorges and re-taking Iasus in 412 BC.