Nicasius of Rouen was a third-century prelate and martyr who lived in Gaul. Often recognized as the Apostle of the Vexin, he is sometimes identified as the first bishop of Rouen, though some historical lists suggest he preceded Saint Mellon. He is traditionally associated with Denis of Paris and is credited with evangelizing various locations along the Seine, including Mousseaux and Rolleboise. His missionary work concluded when he was beheaded alongside two companions near the Epte river in Gasny.