Marie-Théodor Ratisbonne, N.D.S., (December 28, 1802 – January 10, 1884) was a French Jewish convert to the Catholic Church, who became a priest and missionary and who later founded the Congregation of Our Lady of Sion. He was the brother of Marie-Alphonse Ratisbonne, who joined him in this effort. He was baptized in 1826, at which time he added the name Marie (Mary) to his given name, and was ordained as a priest in 1830.