In early 138, the designated heir Lucius Aelius #Caesar died unexpectedly. #Hadrian, already severely ill at this point, responded by adopting #Antoninus and naming him successor. Hadrian seemed to have seen Antoninus as a mere placeholder.

At some date between 110 and 115, #Antoninus married Annia Galeria Faustina (also known as Faustina the Elder to distinguish her from her daughter). A member of a #Roman upper-class family, she was one of #Rome's richest heiresses at the time and a niece of #Hadrian.