|Millennium:||1st millennium BC|
|100 BC by topic|
|Gregorian calendar||100 BC
|Ab urbe condita||654|
|Ancient Egypt era||XXXIII dynasty, 224|
|- Pharaoh||Ptolemy X Alexander, 8|
|Ancient Greek era||170th Olympiad (victor)¹|
|Chinese calendar||庚辰年 (Metal Dragon)
2597 or 2537
— to —
辛巳年 (Metal Snake)
2598 or 2538
|Coptic calendar||−383 – −382|
|Ethiopian calendar||−107 – −106|
|- Vikram Samvat||−43 – −42|
|- Shaka Samvat||N/A|
|- Kali Yuga||3001–3002|
|Iranian calendar||721 BP – 720 BP|
|Islamic calendar||743 BH – 742 BH|
|Minguo calendar||2011 before ROC
|Seleucid era||212/213 AG|
|Thai solar calendar||443–444|
|Wikimedia Commons has media related to 100 BC.|
Year 100 BC was a year of the pre-Julian Roman calendar. At the time it was known as the Year of the Consulship of Marius and Flaccus (or, less frequently, year 654 Ab urbe condita). The denomination 100 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 Valerius Flaccus, Gaius Marius (Marius's sixth consulship).
- Manius Aquillius celebrates an ovation for victories in the Second Servile War.
- Lucius Appuleius Saturninus, a tribune, passes a law to redistribute land to military veterans. The law requires that all senators swear to abide by it. Quintus Caecilus Metellus Numidicus refuses and is exiled. He goes to Rhodes to study philosophy.
- December – Saturninus stands for consul for the following year. A rival candidate, Gaius Memmius, is found murdered by agents of Saturninus, who is declared a public enemy by the Senate. Marius, as consul, defeats his former ally in battle in the Forum. Saturninus and his followers surrender on condition that their lives are spared, but they are stoned to death with roof tiles by renegade senators.
- The building of the Sanctuary of Fortuna Primigenia, Palestrina, Italy, is begun. The model of it is now kept at the Museo Archeologico Nazionale, Italy (approximate date).
- Tigranes II of Armenia is placed on Armenian throne by the Parthians in exchange for the cession of "seventy valleys". (approximate date)
- Peasants revolt under Emperor Wu of Han. The Great Wall is extended out into the Gobi Desert, and sections of the wall are detached with signalling towers.
- Gandhara and Punjab are ruled by the Indo-Greek king Demetrius III Aniketos.
- A History of China is written by Sima Qian (approximate date).
- Olmec III period ends in Southeastern Mexico.
- July 13 – Julius Caesar, Roman general and politician (d. 44 BC)
- Titus Labienus, Caesar's chief lieutenant in the conquest of Gaul (d. 45 BC)
- Cornelia Africana, mother of Tiberius Gracchus (b. c. 190 BC)
- Lucius Appuleius Saturninus, Roman politician
- Theodosius of Bithynia, Greek astronomer and mathematician (b. c. 160 BC)