Titus Aelius Hadrianus Antoninus Pius (19 September AD 86 – 7 March 161) was Roman emperor from AD 138 to 161. He was the fourth of the Five Good Emperors from the Nerva–Antonine dynasty.

Born into a senatorial family, Antoninus held various offices during the reign of Emperor Hadrian. He married Hadrian's niece Faustina, and Hadrian adopted him as his son and successor shortly before his death. Antoninus acquired the cognomen Pius after his accession to the throne, either because he compelled the Senate to deify his adoptive father, or because he had saved senators sentenced to death by Hadrian in his later years.  His reign is notable for the peaceful state of the Empire, with no major revolts or military incursions during this time. A successful military campaign in southern Scotland early in his reign resulted in the construction of the Antonine Wall.

Antoninus was an effective administrator, leaving his successors a large surplus in the treasury, expanding free access to drinking water throughout the Empire, encouraging legal conformity, and facilitating the enfranchisement of freed slaves. He died in his sleep of illness in AD 161 and was succeeded by his adopted sons Marcus Aurelius and Lucius Verus as co-emperors. His death closed out the longest reign since Augustus (surpassing Tiberius by a couple of months).  His record for the second-longest reign would be unbeaten for 168 years, until 329 when it was surpassed by Constantine the Great. 

Antoninus was an effective administrator, leaving his successors a large surplus in the treasury, expanding free access to drinking water throughout the Empire, encouraging legal conformity, and facilitating the enfranchisement of freed slaves. He died of illness in AD 161 and was succeeded by his adopted sons Marcus Aurelius and Lucius Verus as co-emperors.