Roman emperor Hadrian adopted Antoninus Pius as his son and successor following the death of Hadrian's first adopted son Lucius Aelius.

Year 138 (CXXXVIII) was a common year starting on Tuesday of the Julian calendar. At the time, it was known as the Year of the Consulship of Niger and Camerinus. 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.

Hadrian was Roman emperor from 117 to 138. Hadrian was born in Italica, close to modern Seville in Spain, an Italic settlement in Hispania Baetica; his branch of the Aelia gens, the Aeli Hadriani, came from the town of Hadria in eastern Italy. He was a member of the Nerva–Antonine dynasty.

Titus Aelius Hadrianus Antoninus Pius was Roman emperor from AD 138 to 161. He was the fourth of the Five Good Emperors from the Nerva–Antonine dynasty.

Lucius Aelius Caesar was the father of Emperor Lucius Verus. In 136, he was adopted by the reigning emperor Hadrian and named heir to the throne. He died before Hadrian and thus never became emperor. After Lucius' death, he was replaced by Antoninus Pius, who succeeded Hadrian the same year.