|Millennium:||1st millennium BC|
|157 BC by topic|
|Gregorian calendar||157 BC|
|Ab urbe condita||597|
|Ancient Egypt era||XXXIII dynasty, 167|
|- Pharaoh||Ptolemy VI Philometor, 24|
|Ancient Greek era||155th Olympiad, year 4|
|Balinese saka calendar||N/A|
|Chinese calendar||癸未年 (Water Goat)|
2540 or 2480
— to —
甲申年 (Wood Monkey)
2541 or 2481
|Coptic calendar||−440 – −439|
|Ethiopian calendar||−164 – −163|
|- Vikram Samvat||−100 – −99|
|- Shaka Samvat||N/A|
|- Kali Yuga||2944–2945|
|Iranian calendar||778 BP – 777 BP|
|Islamic calendar||802 BH – 801 BH|
|Minguo calendar||2068 before ROC|
|Seleucid era||155/156 AG|
|Thai solar calendar||386–387|
−30 or −411 or −1183
— to —
−29 or −410 or −1182
Year 157 BC was a year of the pre-Julian Roman calendar. At the time it was known as the Year of the Consulship of Caesar and Orestes (or, less frequently, year 597 Ab urbe condita) and the Seventh Year of Houyuan. The denomination 157 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 Carthaginians, prevented by their treaty with Rome from engaging in armed resistance, but equally guaranteed against any loss of territory, appeal to Rome against the depredations of King Masinissa of Numidia. The Roman censor Marcus Porcius Cato heads a commission which arbitrates a truce between Carthage and her former ally, Masinissa.
- During his time in Carthage, Cato is so struck by the evidence of Carthaginian prosperity that he is convinced that the security of Rome now depends on the annihilation of Carthage. From this time on, Cato keeps repeating the cry "Ceterum censeo Carthaginem esse delendam" ("Moreover, I advise that Carthage must be destroyed") at the end of all his speeches, no matter what subject they concern.
- After Ariarathes V has been deposed from the Cappadocian throne by the Seleucid king Demetrius I Soter and has fled to Rome, the new king of Cappadocia, Orophernes, sends two ambassadors to Rome to join the Seleucid emissaries of Demetrius in opposing Ariarathes V's return to power. Despite their efforts, Ariarathes V is restored to his throne by the Romans. However, Rome allows Orophernes to reign jointly with him. The joint government, however, does not last long, as Ariarathes V becomes sole king of Cappadocia shortly afterwards.
- Jonathan Maccabeus is recognised by the Seleucids as a minor king within their dominions.
- 30 July – Wu of Han, who will be emperor of the Chinese Han dynasty from 141 BC (d. 87 BC)
- Gaius Marius, Roman general and politician who will be elected consul seven times; he will also introduce major reforms to the Roman army, authorising recruitment of landless citizens and reorganising the structure of the legions (d. 86 BC)
- Sanatruces (also known as Sinatruces or Sanatruk), King of Parthia who will rule the Parthian Empire from around 77 BC (approximate date) (d. c. 70 BC)
- July 6 – Wen of Han, Emperor of the Chinese Han dynasty since 180 BC (b. 200 BC)