|Millennium:||1st millennium BC|
|90 BC by topic|
|Gregorian calendar||90 BC|
|Ab urbe condita||664|
|Ancient Egypt era||XXXIII dynasty, 234|
|- Pharaoh||Ptolemy X Alexander, 18|
|Ancient Greek era||172nd Olympiad, year 3|
|Balinese saka calendar||N/A|
|Chinese calendar||庚寅年 (Metal Tiger)|
2607 or 2547
— to —
辛卯年 (Metal Rabbit)
2608 or 2548
|Coptic calendar||−373 – −372|
|Ethiopian calendar||−97 – −96|
|- Vikram Samvat||−33 – −32|
|- Shaka Samvat||N/A|
|- Kali Yuga||3011–3012|
|Iranian calendar||711 BP – 710 BP|
|Islamic calendar||733 BH – 732 BH|
|Minguo calendar||2001 before ROC|
|Seleucid era||222/223 AG|
|Thai solar calendar||453–454|
37 or −344 or −1116
— to —
38 or −343 or −1115
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 (approximate date)
- Pasherienptah III, Egyptian high priest (d. 41 BC)
- Antiochus X Eusebes, Seleucid king (approximate date)
- Dionysios Trax, Greek grammarian (b. 170 BC)
- Li Yannian, Chinese musician (executed by Wu of Han)
- Pharnajom, king of Iberia (modern Georgia)