|Millennium:||1st millennium BC|
|20 BC by topic|
|Gregorian calendar||20 BC|
|Ab urbe condita||734|
|Ancient Greek era||190th Olympiad (victor)¹|
|Balinese saka calendar||N/A|
|Chinese calendar||庚子年 (Metal Rat)|
2677 or 2617
— to —
辛丑年 (Metal Ox)
2678 or 2618
|Coptic calendar||−303 – −302|
|Ethiopian calendar||−27 – −26|
|- Vikram Samvat||37–38|
|- Shaka Samvat||N/A|
|- Kali Yuga||3081–3082|
|Iranian calendar||641 BP – 640 BP|
|Islamic calendar||661 BH – 660 BH|
|Julian calendar||20 BC|
|Minguo calendar||1931 before ROC|
|Seleucid era||292/293 AG|
|Thai solar calendar||523–524|
107 or −274 or −1046
— to —
108 or −273 or −1045
Year 20 BC was either a common year starting on Wednesday or Thursday or a leap year starting on Tuesday, Wednesday or Thursday (link will display the full calendar) of the Julian calendar (the sources differ, see leap year error for further information) and a common year starting on Tuesday of the Proleptic Julian calendar. At the time, it was also known as the Year of the Consulship of Appuleius and Nerva (or, less frequently, year 734 Ab urbe condita). The denomination 20 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.
- May 12 – Emperor Augustus Caesar negotiates a peace with Parthia, making Armenia a buffer zone between the two major powers. The captured eagles of Marcus Licinius Crassus and Mark Antony are returned.
- Based on the scenes and the style of the work, the Portland Vase is believed to have been made in Alexandria some time between this year and AD 100.
- King Herod the Great begins renovation of the Temple in Jerusalem.
- Maison Carrée, Nîmes, France, is built (approximate date).
- Marcus Verrius Flaccus' De verborum significatu is published. It is one of the first great dictionaries in history.
- June 3 – Lucius Aelius Sejanus, advisor of Tiberius (d. AD 31)
- Gaius Caesar, grandson of Augustus Caesar (d. AD 4)
- Lucius Antonius, grandson of Mark Antony d. AD 25)
- Philo of Alexandria, Jewish philosopher (d. AD 50)
- Artavasdes I, king of Media Atropatene (approximate date)
- Artaxias II, king of Armenia (Artaxiad Dynasty)
- Fuzhulei Ruodi, ruler of the Xiongnu Empire
- Mirian II (or Mirvan), king of Iberia (Georgia)
- Mithridates II, king of Commagene (Armenia)