Diodorus Siculus /ˌdaɪəˈdɔːrəsˈsɪkjʊləs/ (Ancient Greek: Διόδωρος Σικελιώτης Diodoros Sikeliotes) was a Greek historian, who wrote works of history between 60 and 30 BC. He is known for the monumental universal history Bibliotheca historica. According to Diodorus' own work, he was born at Agyrium in Sicily (now called Agira). With one exception, antiquity affords no further information about Diodorus' life and doings beyond what is to be found in his own work. Only Jerome, in his Chronicon under the "year of Abraham 1968" (i.e., 49 BC), writes, "Diodorus of Sicily, a writer of Greek history, became illustrious". His English translator, Charles Henry Oldfather, remarks on the "striking coincidence" that one of only two known Greek inscriptions from Agyrium (I.G. XIV, 588) is the tombstone of one "Diodorus, the son of Apollonius".

Diodorus Siculus, Greek historian of Agyrium in Sicily, ca. 80–20 BCE, wrote forty books of world history, called Library of History, in three parts: mythical history of peoples, non-Greek and Greek, to the Trojan War; history to Alexander's death (323 BCE); history to 54 BCE. Of this we have complete Books I–V (Egyptians, Assyrians, Ethiopians, Greeks) and Books XI–XX (Greek history 480–302 BCE); and fragments of the rest. He was an uncritical compiler, but used good sources and reproduced them faithfully. He is valuable for details unrecorded elsewhere, and as evidence for works now lost, especially writings of Ephorus, Apollodorus, Agatharchides, Philistus, and Timaeus.

This is the twelfth and final volume of the Loeb series which includes an index and supplemental material for the earlier volumes.