St. Francis de Sales Church (the Oratory of Saint Francis de Sales) is a Roman Catholic Oratory located in south St. Louis, Missouri. 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." The historic main church was designed in the neo-Gothic style. Its stained glass windows were crafted by the St. Louis glazier Emil Frei, Sr.