Pope Saint Pius I was Bishop of Rome, according to the Annuario Pontificio, from 142 or 146 to 157 or 161, respectively. Others suggest that his pontificate was perhaps from 140 to 154.

Pius is believed to have been born at Aquileia, in Northern Italy, during the late 1st century. His father was called "Rufinus", who was also said to be of Aquileia according to the Liber Pontificalis.

It is stated in the 2nd century Muratorian Canon, and in the Liberian Catalogue, that he was the brother of Hermas, author of the text known as The Shepherd of Hermas. The writer of the later text identifies himself as a former slave. This has led to speculation that both Hermas and Pius were freedmen.

St Pius I governed the Church in the middle of the 2nd century during the reigns of the Emperors Antoninus Pius and Marcus Aurelius. He was the ninth successor of Saint Peter. He decreed that Easter should only be kept on a Sunday. Although being credited with ordering the publication of the Liber Pontificalis, compilation of that document was not started before the beginning of the 6th century. He is said to have built one of the oldest churches in Rome, Santa Pudenziana.