Diocles of Peparethus was a historian from the Greek island of Peparethus. His works are lost, but they included histories of Persia and Rome: Quintus Fabius Pictor

Quintus Fabius Pictor

Quintus Fabius Pictor was one of the earliest Roman historians and considered the first of the annalists. A member of the Fabii gens, he was the grandson of Gaius Fabius Pictor, a painter . He was a senator who fought against the Gauls in 225 BC, and against Carthage in the Second Punic War...

 and Plutarch

Plutarch

Plutarch then named, on his becoming a Roman citizen, Lucius Mestrius Plutarchus , c. 46 – 120 AD, was a Greek historian, biographer, essayist, and Middle Platonist known primarily for his Parallel Lives and Moralia...

 acknowledge their debts to the latter as a source for their histories of early Rome, its native traditions and ancestral Greek connections. Fabius' work survives only as a brief but historically significant catalogue summary. Plutarch seems to have relied on Fabius' history but acknowledges Diocles as its basis and authority. Diocles' own sources are unknown. He may have had access to Roman sources and traditions on which he foisted Greek interpretations and interpolations.
Little else is known of Diocles. He appears to have been a figure of note, well travelled, and abstemious; Athenaeus

Athenaeus

Athenaeus , of Naucratis in Egypt, Greek rhetorician and grammarian, flourished about the end of the 2nd and beginning of the 3rd century AD...

cites Demetrius of Scepsis to attest that Diocles "drank cold water to the day of his death".