St. Francis de Sales Church is a Roman Catholic Oratory located in south St. Louis, Missouri, United States. It is the second largest church in the Archdiocese of St. Louis after the cathedral-basilica. The church is popularly known as the "Cathedral of South St. Louis".

(Greek: εὐκτηρίου) In the canon law of the Catholic Church, an oratory is a place which is set aside by permission of an ordinary for divine worship, for the convenience of some community or group of the faithful who assemble there, but to which other members of the faithful may have access with the consent of the competent superior. The word oratory comes from the Latin verb orare, to pray.