|Millennium:||1st millennium BC|
|123 BC by topic|
|Gregorian calendar||123 BC|
|Ab urbe condita||631|
|Ancient Egypt era||XXXIII dynasty, 201|
|- Pharaoh||Ptolemy VIII Physcon, 23|
|Ancient Greek era||164th Olympiad, year 2|
|Balinese saka calendar||N/A|
|Chinese calendar||丁巳年 (Fire Snake)|
2574 or 2514
— to —
戊午年 (Earth Horse)
2575 or 2515
|Coptic calendar||−406 – −405|
|Ethiopian calendar||−130 – −129|
|- Vikram Samvat||−66 – −65|
|- Shaka Samvat||N/A|
|- Kali Yuga||2978–2979|
|Iranian calendar||744 BP – 743 BP|
|Islamic calendar||767 BH – 766 BH|
|Minguo calendar||2034 before ROC|
|Seleucid era||189/190 AG|
|Thai solar calendar||420–421|
4 or −377 or −1149
— to —
5 or −376 or −1148
Year 123 BC was a year of the pre-Julian Roman calendar. At the time it was known as the Year of the Consulship of Balearicus and Flamininus (or, less frequently, year 631 Ab urbe condita) and the Sixth Year of Yuanshuo. The denomination 123 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.
- Gaius Gracchus elected Roman tribune for the first time. He waited until after his re-election the following year before pushing forward the various civil and agrarian reforms that his brother championed in 133 BC.
- Aix-en-Provence founded under the name of Aquae Sextiae by the Roman consul Sextius Calvinus.
- Quintus Caecilius Metellus conquers the Balearic Islands for Rome, for which he earns the cognomen "Balearicus." He settles 3,000 Roman and Iberian colonists on the islands and founds the cities of Palma and Pollentia.