|Millennium:||1st millennium BC|
|86 BC by topic|
|Gregorian calendar||86 BC|
|Ab urbe condita||668|
|Ancient Egypt era||XXXIII dynasty, 238|
|- Pharaoh||Ptolemy IX Lathyros, 3|
|Ancient Greek era||173rd Olympiad, year 3|
|Balinese saka calendar||N/A|
|Chinese calendar||甲午年 (Wood Horse)|
2611 or 2551
— to —
乙未年 (Wood Goat)
2612 or 2552
|Coptic calendar||−369 – −368|
|Ethiopian calendar||−93 – −92|
|- Vikram Samvat||−29 – −28|
|- Shaka Samvat||N/A|
|- Kali Yuga||3015–3016|
|Iranian calendar||707 BP – 706 BP|
|Islamic calendar||729 BH – 728 BH|
|Minguo calendar||1997 before ROC|
|Seleucid era||226/227 AG|
|Thai solar calendar||457–458|
41 or −340 or −1112
— to —
42 or −339 or −1111
Year 86 BC was a year of the pre-Julian Roman calendar. At the time it was known as the Year of the Consulship of Cinna and Marius/Flaccus (or, less frequently, year 668 Ab urbe condita). The denomination 86 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.
- First Mithridatic War
- March 1 – Sulla captures Athens from the Pontic army, removing the tyrant Aristion.
- Lucius Licinius Lucullus decisively defeats the Mithridatic fleet in the Battle of Tenedos.
- The Roman forces of Lucius Cornelius Sulla defeat the Pontic forces of Archelaus in the Battle of Chaeronea.
- The Dardani ally with Pontus and are defeated by Sulla soon after.
- October 1 – Sallust, Roman historian (d. 34 BC)
- Faustus Cornelius Sulla, Roman senator, son of the dictator Lucius Cornelius Sulla (d. 46 BC)
- Fausta Cornelia Sulla, twin sister of Faustus Cornelius Sulla, wife of Gaius Memmius and later of Titus Annius Milo Papianus
- January 13 – Gaius Marius, Roman general and politician (b. 157 BC)
- March 1 – Aristion, Greek philosopher and tyrant
- Jin Midi, Chinese politician and co-regent (b. 134 BC)
- Sima Qian, Chinese historian (b. 145 BC)