Roman emperor from AD 138 to 161. Mild-mannered and capable, he was the fourth of the “five good emperors” who guided the empire through an 84-year period (96–180) of internal peace and prosperity. His family originated in Gaul, and his father and grandfathers had all been consuls.

After serving as consul in 120, Antoninus was assigned by the emperor Hadrian (ruled 117–138) to assist with judicial administration in Italy. He governed the province of Asia (c. 134) and then became an adviser to the Emperor. In 138 Antoninus was adopted by Hadrian and designated as his successor. Hadrian specified that two men—the future emperors Marcus Aurelius and Lucius Verus—were to succeed Antoninus. Upon acceding to power, Antoninus persuaded a reluctant Senate to offer the customary divine honours to Hadrian. For this, and possibly other such dutiful acts, he was given the surname Pius by the Senate. When his wife, Faustina, died in late 140 or early 141 he founded in her memory the Puellae Faustinianae, a charitable institution for the daughters of the poor.

References to Antoninus in 2nd-century literature are exceptionally scanty; it is certain that few striking events occurred during his 23-year reign. A rebellion in Roman Britain was suppressed, and in 142 a 36-mile (58-kilometre) garrisoned barrier—called the Antonine Wall—was built to extend the Roman frontier some 100 miles north of Hadrian's Wall (q.v.). Antoninus' armies contained revolts in Mauretania, Germany, Dacia, and Egypt.

The feeling of well-being that pervaded the empire under Antoninus is reflected in the celebrated panegyric by the orator Aelius Aristides in 143–144. After Antoninus' death, however, the empire suffered invasion by hostile tribes, followed by severe civil strife.