Lucius Cassius Dio Cocceianus (Ancient Greek: Δίων ὁ Κάσσιος, c. AD 150 – 235 , known in English as Cassius Dio, Dio Cassius, or Dio (Dione. lib) was a Roman consul and a noted historian writing in Greek. Dio published a history of Rome in 80 volumes, beginning with the legendary arrival of Aeneas in Italy through the subsequent founding of Rome (753 BC), the formation of the Republic (509 BC), and the creation of the Empire (31 BC), up to AD 229; a period of about 1,400 years. Of the 80 books, written over 22 years, many survive into the modern age intact or as fragments, providing modern scholars with a detailed perspective on Roman history.

Cassius Dio was the son of Cassius Apronianus, a Roman senator. He was born and raised at Nicaea in Bithynia. Byzantine tradition holds that Dio's mother was the daughter or sister of the Greek orator and philosopher Dio Chrysostom; this relationship has been disputed. His praenomen is usually held to have been Lucius, but a Macedonian inscription published in 1970 shows it as Cl., presumably Claudius. Although a Roman citizen, he wrote in Greek. Dio always maintained a love for his hometown of Nicaea, calling it 'his home', as opposed to his description of his villa in Italy ('my residence in Italy').