|Millennium:||1st millennium BC|
|121 BC by topic|
|Gregorian calendar||121 BC|
|Ab urbe condita||633|
|Ancient Egypt era||XXXIII dynasty, 203|
|- Pharaoh||Ptolemy VIII Physcon, 25|
|Ancient Greek era||164th Olympiad, year 4|
|Balinese saka calendar||N/A|
|Chinese calendar||己未年 (Earth Goat)|
2576 or 2516
— to —
庚申年 (Metal Monkey)
2577 or 2517
|Coptic calendar||−404 – −403|
|Ethiopian calendar||−128 – −127|
|- Vikram Samvat||−64 – −63|
|- Shaka Samvat||N/A|
|- Kali Yuga||2980–2981|
|Iranian calendar||742 BP – 741 BP|
|Islamic calendar||765 BH – 764 BH|
|Minguo calendar||2032 before ROC|
|Seleucid era||191/192 AG|
|Thai solar calendar||422–423|
6 or −375 or −1147
— to —
7 or −374 or −1146
Year 121 BC was a year of the pre-Julian Roman calendar. At the time it was known as the Year of the Consulship of Opimius and Allobrogicus (or, less frequently, year 633 Ab urbe condita) and the Second Year of Yuanshou. The denomination 121 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.
- The Roman Senate passes the motion senatus consultum ultimum, which the consul Lucius Opimius interprets as giving him unlimited power to preserve the Republic. He gathers an armed force of Senators and their supporters to confront Gaius Gracchus. A pitched battle is fought inside Rome, resulting in the death of Gracchus and many of his followers.
- A tribunal is established in Rome that executes 3,000 followers of Gracchus.
- Consul Quintus Fabius Maximus, allied with the Aedui, defeated the Arverni and Allobroges in Transalpine Gaul, thus establishing the province for Rome.
- The finest vintage of Falernian wine, known as the Opimian vintage, was bottled from vines grown on Mt Falernus between Latium and Campania.
- Led by General Huo Qubing, the Chinese defeat the Xiongnu,killing more than 40000 Xiongnu soldiers，including seven xiongnu princes.
- Gaius Gracchus, Roman reformer (b. 154 BC)
- M. Fulvius Flaccus (b. 154 BC)
- Cleopatra Thea (failed attempt to poison her stepson) (b. c. 164 BC)