Darius I (Old Persian: 𐎭𐎠𐎼𐎹𐎺𐎢𐏁 Dārayavaʰuš; Greek: Δαρεῖος Dareios; c. 550 – 486 BCE), commonly known as Darius the Great, was a Persian ruler who served as the third King of Kings of the Achaemenid Empire, reigning from 522 BCE until his death in 486 BCE. He ruled the empire at its territorial peak, when it included much of Western Asia, parts of the Balkans (Thrace–Macedonia and Paeonia) and the Caucasus, most of the Black Sea's coastal regions, Central Asia, the Indus Valley in the far east, and portions of North Africa and Northeast Africa including Egypt (Mudrâya), eastern Libya, and coastal Sudan.

Darius ascended the throne by overthrowing the legitimate Achaemenid monarch Bardiya, whom he later fabricated to be an imposter named Gaumata. The new king met with rebellions throughout his kingdom and quelled them each time; a major event in Darius' life was his expedition to subjugate Greece and punish Athens and Eretria for their participation in the Ionian Revolt. Although his campaign ultimately resulted in failure at the Battle of Marathon, he succeeded in the re-subjugation of Thrace and expanded the Achaemenid Empire through his conquests of Macedon, the Cyclades and the island of Naxos as well as the sacked Greek city of Eretria.

Darius organized the empire by dividing it into administrative provinces that were governed by satraps. He organized Achaemenid coinage as a new uniform monetary system, and made Aramaic a co-official language of the empire alongside Persian. He also put the empire in better standing by building roads and introducing standard weighing and measuring systems. Through these changes, the Achaemenid Empire became centralized and unified. Darius worked on other construction projects throughout the empire, primarily focusing on Susa, Pasargadae, Persepolis, Babylon and Egypt. He had the cliff-face Behistun Inscription carved at Mount Behistun to record his conquests, which would later become an important testimony of the Old Persian language.

Darius is mentioned in the books of Haggai, Zechariah and Ezra–Nehemiah of the Hebrew Bible.