|Millennium:||1st millennium BC|
|138 BC by topic|
|Gregorian calendar||138 BC|
|Ab urbe condita||616|
|Ancient Egypt era||XXXIII dynasty, 186|
|- Pharaoh||Ptolemy VIII Physcon, 8|
|Ancient Greek era||160th Olympiad, year 3|
|Balinese saka calendar||N/A|
|Chinese calendar||壬寅年 (Water Tiger)|
2559 or 2499
— to —
癸卯年 (Water Rabbit)
2560 or 2500
|Coptic calendar||−421 – −420|
|Ethiopian calendar||−145 – −144|
|- Vikram Samvat||−81 – −80|
|- Shaka Samvat||N/A|
|- Kali Yuga||2963–2964|
|Iranian calendar||759 BP – 758 BP|
|Islamic calendar||782 BH – 781 BH|
|Minguo calendar||2049 before ROC|
|Seleucid era||174/175 AG|
|Thai solar calendar||405–406|
−11 or −392 or −1164
— to —
−10 or −391 or −1163
Year 138 BC was a year of the pre-Julian Roman calendar. At the time it was known as the Year of the Consulship of Serapio and Callaicus (or, less frequently, year 616 Ab urbe condita) and the Third Year of Jianyuan. The denomination 138 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.
- Galaestes revolts.
- Zhang Qian begins his explorations in central Asia for Chinese emperor Han Wu Di.
- The first Chinese diplomatic mission to the Fergana valley, led by Chang Chien, is sent.
- Han dynasty China intervenes in a war between the Minyue and Eastern Ou during its expansion southward.
Arts and sciences
- Hymn to Apollo is written and inscribed on stone in Delphi; it is the earliest surviving notated music, in a substantial and legible fragment, in the western world.
- Lucius Cornelius Sulla, Roman general and statesman (d. 78 BC)
- Phaedrus the Epicurean, Greek scholar and philosopher
- Attalus II Philadelphus, king of Pergamon (b. 220 BC)
- Diodotus Tryphon, king of the Seleucid Empire
- Mithridates I, king of Parthia (b. c. 195 BC)