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|Ab urbe condita||891|
|Balinese saka calendar||59–60|
|Chinese calendar||丁丑年 (Fire Ox)|
2834 or 2774
— to —
戊寅年 (Earth Tiger)
2835 or 2775
|Coptic calendar||−146 – −145|
|- Vikram Samvat||194–195|
|- Shaka Samvat||59–60|
|- Kali Yuga||3238–3239|
|Iranian calendar||484 BP – 483 BP|
|Islamic calendar||499 BH – 498 BH|
|Minguo calendar||1774 before ROC|
|Seleucid era||449/450 AG|
|Thai solar calendar||680–681|
264 or −117 or −889
— to —
265 or −116 or −888
Year 138 (CXXXVIII) was a common year starting on Tuesday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Niger and Camerinus (or, less frequently, year 891 Ab urbe condita). The denomination 138 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.
- February 25 – Emperor Hadrian makes Antoninus Pius his successor, on condition that he adopt Marcus Aurelius and Lucius Verus.
- July 10 – Hadrian dies after a heart failure at Baiae, and is buried at Rome in the Gardens of Domitia beside his wife, Vibia Sabina.
- Antoninus Pius succeeds Hadrian as Roman Emperor, and asks the Senate to confer divine honors for Hadrian.
- Construction begins on the Theater of Philadelphia (Amman).
- Hadrian's Villa, Tivoli, Italy, is finished.
- The silver content of the Roman denarius falls to 75 percent under Emperor Antoninus Pius, down from 87 percent under Hadrian.
- Han Zhidi, Chinese emperor of the Han Dynasty (d. 146)
- Marcus Ummidius Quadratus, Roman politician (d. 182)
- January 1 – Lucius Aelius Caesar, Roman politician and adopted son of Hadrian (b. 101)
- July 10 – Hadrian, Roman emperor (b. AD 76)
- Zenobius, Greek sophist and writer (b. 117)