Josef Fessler (1813–1872) was Roman Catholic Bishop of Sankt Pölten in Austria, a secretary of the First Vatican Council and an authority on patristics.

Fessler was informed of his appointment on 27 March, and as the pope wished him to come with all speed to Rome, he arrived there on 8 July, after hastily dispatching the business of his diocese.

The Catholic Encyclopedia describes Fessler's work as secretary as giving "universal satisfaction", praising his "vast and intimate acquaintance with the Church Fathers and with ecclesiastical history" and his "thorough knowledge of canon law."

In this brochure, Fessler argued for a definition of infallibility based on the writings of Italian Ultramontane theologians including Robert Bellarmine, Pietro Ballerini, and Giovanni Perrone.

Karl Josef von Hefele, the Bishop of Rottenburg, who had previously opposed the doctrine of infaillibility, wrote in support of Fessler's position, and Pius IX published a Brief approving it.