|Millennium:||1st millennium BC|
|91 BC by topic|
Year 91 BC was a year of the pre-Julian Roman calendar. At the time it was known as the Year of the Consulship of Philippus and Caesar (or, less frequently, year 663 Ab urbe condita). The denomination 91 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: Sextus Julius Caesar and Lucius Marcius Philippus
- The tribune Marcus Livius Drusus proposes extending Roman citizenship to allied Italian cities, but is assassinated, leading to the Social War.
- Liu Ju, crown prince of the Han Dynasty, revolts against his father, Emperor Wu, and his witchcraft trials. After his rebellion fails, he hangs himself.
- Records of the Grand Historian compiled by Sima Qian.
- Nicomedes II, king of Bithynia
- Marcus Livius Drusus, murdered
- Lucius Licinius Crassus, Roman consul and orator (b. 140 BC)
- Liu Ju, crown prince of the Han Dynasty, suicide (b. 128 BC)
- Empress Wei Zifu