|Millennium:||1st millennium BC|
|91 BC by topic|
|Gregorian calendar||91 BC|
|Ab urbe condita||663|
|Ancient Egypt era||XXXIII dynasty, 233|
|- Pharaoh||Ptolemy X Alexander, 17|
|Ancient Greek era||172nd Olympiad, year 2|
|Balinese saka calendar||N/A|
|Chinese calendar||己丑年 (Earth Ox)|
2606 or 2546
— to —
庚寅年 (Metal Tiger)
2607 or 2547
|Coptic calendar||−374 – −373|
|Ethiopian calendar||−98 – −97|
|- Vikram Samvat||−34 – −33|
|- Shaka Samvat||N/A|
|- Kali Yuga||3010–3011|
|Iranian calendar||712 BP – 711 BP|
|Islamic calendar||734 BH – 733 BH|
|Minguo calendar||2002 before ROC|
|Seleucid era||221/222 AG|
|Thai solar calendar||452–453|
36 or −345 or −1117
— to —
37 or −344 or −1116
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.
- Quintus Caecilius Metellus Numidicus, Roman politician
- Liu Ju, crown prince of the Han Dynasty (b. 128 BC)
- Lucius Licinius Crassus, Roman consul (b. 140 BC)
- Marcus Livius Drusus, Roman politician
- Wei Zifu, empress of the Han Dynasty