Pius I was pope, perhaps from 140 to 154, though the Vatican's "Annuario Pontificio" (2003) lists 142 or 146 to 157 or 161. He is believed to have been born at Aquileia, his father being one "Rufinus". Some conjecture that he was a martyr, and he is celebrated as such in the Breviary. Nevertheless, there is little other evidence that supports such a conclusion.

It is stated in the second century Muratorian Canon that he was the brother of Hermas, author of the text 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.

Feast day 11 July.