Cyriacus, or Cyriac, is a Christian martyr who was killed in the persecution of Diocletian. He is one of twenty-seven saints, most of them martyrs, who bear this name, of whom only seven are honoured by a specific mention of their names in the Roman Martyrology.

Of the Saint Cyriacus who, together with Saints Largus and Smaragdus and others (of whom Crescentianus, Memmia and Juliana are mentioned in the Roman Martyrology), is venerated on 8 August, all that is known with certainty, apart from their names and the fact of their martyrdom, is that they were buried at the seventh milestone of the Via Ostiensis on that date.

However, legend has it that Cyriacus was a Roman nobleman who converted to Christianity as an adult and, renouncing his material wealth, gave it away to the poor. He spent the rest of his life ministering to the slaves who worked in the Baths of Diocletian. Under the reign of Western Roman Emperor Maximian, co-emperor with Diocletian, Cyriacus was tortured and put to death, beheaded in 303 on the Via Salaria, where he was subsequently buried. With him were martyred his companions Largus and Smaragdus, and twenty others, including Crescentianus, Sergius, Secundus, Alban, Victorianus, Faustinus, Felix, Sylvanus, and four women: Memmia, Juliana, Cyriacides, and Donata.