The former French Catholic diocese of Saint-Omer existed from 1559 until the French Revolution. Its see at Saint-Omer, in the modern department of Pas-de-Calais, was created as a reaction to the destruction of the see of Thérouanne, by military action in the wars of the Emperor Charles V. It then became a suffragan of the Archdiocese of Cambrai in 1559.
By the Concordat of 1801, the diocese of Saint-Omer was united with the diocese of Arras and the diocese of Boulogne, to form an enlarged diocese of Arras.