Bertin of Sithiu was a Frankish monk who became the abbot under whom the monastery of Sithiu, in the far north of Gaul, rose to become one of the leading monastic centres of northern Europe. By tradition he was born near Constance, in the territory of Alamannia, around the year 615, and entered the monastery of Luxeuil while still young, where he was formed under the austere rule associated with St. Columbanus. He is commemorated on September 5.
About the year 638 Bertin set out with two companions, Mummolin and Ebertram, for the northern reaches of France to assist St. Omer, Bishop of Therouanne, in the evangelization of the Morini, a people among whom paganism still prevailed. The missionaries settled at a place called Sithiu, on land that had been given to Bishop Omer by a nobleman named Adrowald; the town of Saint-Omer later grew up in the same region. Mummolin governed the monastic community as abbot until he was made Bishop of Noyon, about the year 659, after which Bertin assumed the abbacy.
Under Bertin's long government the community flourished, and the sources record that more than 150 monks came to live under his rule. Among them was St. Winnoc, who came from Brittany with three companions to join the monastery and assist in the conversion of the surrounding population. As his death approached, Bertin appointed a monk named Rigobert to succeed him. He reposed around the year 709. The monastery, which had first been dedicated to St. Peter, was afterwards rededicated to its second abbot and became known as the Abbey of Saint Bertin; it ranked in importance with the monasteries of Elnon and St. Vaast and remained one of the most influential houses in the north of Europe. He is numbered among the pre-Schism Western saints venerated as Orthodox.