The thesis presents a global analysis of the Vita Beati Patris Nostri Francisci by Thomas of Celano, discovered in a manuscript by the French scholar Jacques Dalarun in 2015. The first chapter is dedicated to a short biographical summary on Saint Francis and the role played by the saint in his era, with particular reference to the situation of the Catholic Church in the Middle Ages. The second chapter focuses on the so-called quaestio franciscana, that is, on the chronological and philological problems that concern the biographical sources on Saint Francis. We focus on the Life of Saint Francis published in 1894 by Paul Sabatier, who marked a turning point in the question of hagiographies and re-evaluated the pre-eminent importance of Thomas of Celano's works in the hagiographic panorama related to Saint Francis. The Vita Beati Patris Nostri Francisci found by Dalarun has naturally reopened the question: the whole chapter three is dedicated to it, in which we talk about the discovery of the so-called Vita brevior and we analyse its content and its historical-biographical and theological value. Vita brevior opens with a letter addressed to Brother Elias, by whom this Vita (an abbreviation of the previous biographies written by Thomas) was evidently requested: the most significant data is that Vita brevior, however, does not seem a simple abbreviation, but a synthesis aimed at focusing attention on the authentic Franciscan spirit. Finally, in the last chapter, the biblical quotations in the text by Thomas are analysed: it is noted that the quotations from the New Testament prevail (in particular from the Gospel of Matthew), which can be explained with the centrality recognized to the Gospel by Saint Francis himself.