|Millennium:||1st millennium BC|
|90 BC by topic|
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Year 90 BC was a year of the pre-Julian Roman calendar. At the time it was known as the Year of the Consulship of Caesar and Lupus (or, less frequently, year 664 Ab urbe condita). The denomination 90 BC for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.
- Consuls: Lucius Julius Caesar and Publius Rutilius Lupus.
- Social War continues: Pompeius Strabo and Gaius Marius distinguish themselves.
- The Etruscans are granted Roman citizenship.
- Corfinium in south-central Italy is the center of a rebellion against Rome.
- The Lex Iulia grants citizenship to all Italians who did not oppose Rome during the Social War.
- Cicero starts to serve in the Roman army.
- Nicomedes IV of Bithynia is defeated in battle by a coalition of Nicomedes' brother Socrates, and Mithridates VI of Pontus. Nicomedes flees to Rome.
- Aulus Hirtius, Roman politician and historian (d. 43 BC)
- Diodorus Siculus, Greek historian (probable date)
- Dionysios Trax, Greek linguist (b. 170 BC)
- Antiochus X Eusebes, king of the Seleucid Empire (approximate date)