Pierre Coustant (born at Compiègne, France, 30 April 1654; died at the Abbey of Saint-Germain-des-Prés, near Paris, 18 October 1721) was a French Benedictine scholar, of the Congregation of Saint-Maur.
After receiving his classical education in the Jesuit College at Compiègne, he entered the Benedictine monastery of Saint-Rémi at Reims as novice at the age of seventeen, and took vows on 12 August 1672.
He also aided his fellow Benedictines Edmond Martène and Robert Morel in making the indexes for the fourth volume containing the commentaries on the Psalms.
The preface is followed by two biographical sketches of the saint, the former of which was composed by Coustant himself from the writings of Hilary, while the latter is a reproduction of the life written by Fortunatus of Poitiers.
It was published in one folio volume at Paris in 1693 and bears the title: St. Hilary, Bishop of Poitiers, complete extant works, sought after by nearly every man-of-letters in the world, now, not with a moderate effort and some a censure and disturbance from the state, is now restored to its true and pious meaning: Finally amended by a comparison of the original books, explained by a variety of readings, enriched by the accession of various treatises.
Immediately upon the publication of Augustine's works in 1700, Coustant was entrusted by his superiors with the editing of a complete collection of the letters of the popes from Clement I to Innocent III (c. 88-1216).
About twelve years later, Ursin Durand undertook to continue the work; in his case the Jansenist controversy in which he became involved prevented the publication of the material he had prepared.
There are extant in the Bibliothèque Nationale at Paris fourteen large folio volumes containing the material gathered by Coustant and his Benedictine continuators.