Mellonius was a 4th-century bishop of Rotomagus, known today as Rouen, in the Roman province of Lugdunensis. While he is recognized as a Catholic saint, his historical existence remains uncertain because his primary biography was written in the 17th century. According to legend, he was born near Cardiff and traveled to Rome, where he was converted to Christianity and ordained by Pope Stephen I. He eventually moved to Gaul to serve as a bishop, following the death of Nicasius.