Saint Denis (also called Dionysius, Dennis, or Denys) is a Christian martyr and saint. In the third century, he was Bishop of Paris. He was martyred in connection with the Decian persecution of Christians, shortly after A.D. 250. After his head was chopped off, Denis is said to have picked it up and walked ten kilometres (six miles), preaching a sermon the entire way, making him one of many cephalophores in hagiology. He is venerated in the Roman Catholic Church as patron of Paris, France and as one of the Fourteen Holy Helpers. The medieval and modern French name "Denis" derives from the ancient name Dionysius.

Gregory of Tours states that Denis was bishop of the Parisii and was martyred by being beheaded by a sword. The earliest document giving an account of his life and martyrdom, the "Passio SS. Dionysii Rustici et Eleutherii" dates from c. 600, is mistakenly attributed to the poet Venantius Fortunatus, and is legendary. Nevertheless, it appears from the Passio that Denis was sent from Italy to convert Gaul in the third century, forging a link with the "apostles to the Gauls" reputed to have been sent out under the direction of Pope Fabian. This was after the persecutions under Emperor Decius had all but dissolved the small Christian community at Lutetia. Denis, with his inseparable companions Rusticus and Eleutherius, who were martyred with him, settled on the Île de la Cité in the River Seine. Roman Paris lay on the higher ground of the Left Bank, away from the river.