Recent French scholarship has begun to examine some of the fundamental concepts that helped to build the French modern state. In his newly edited collective work, Jean-Philippe Genet does not hesitate to call ″truth″ one of the vecteurs de l'idéel in this grand historical process, and various scholars have discussed about different aspects of the growing Foucaultian ″truth regime″ in late medieval France. It is, however, to be admitted that precedent studies did not pay much attention to how exactly this ″truth regime″ was established in the medieval French Kingdom. In fact, as we shall argue, the ideology of truth, originated from the ecclesiastical discourse of reform as a result of Gregory VII's ″papal revolution,″ was officially recognized and absorbed into the ideology of monarchical governance during the reign of Louis IX (Saint Louis) and was further amplified in the process of his canonization.  To capture the concrete path of this intellectual and institutional evolution, this article proposes to examine the legal aspect of the advent of this ″truth regime.″ Constantly seeking ″truth″ is an important element in the making of Louis IX's sanctity and constitutes the essential part of his legal image. Echoing the Church's opposition of the truth against the custom (″Dominus, Ego sum, inquit, veritas. Non dixit, Ego sum consuetudo.″) and its call for abolishing evil customs, the saintly king was portrayed by various historical sources as an earthly promoter of ideal justice and a reforming legislator who was resolute enough in his fight against the chaotic legal order. All his actions and policies drew their inspiration and legitimacy from the idea of pursuing ″truth.″ In this sense, ″truth″ is the key concept which bridges the Church ideology and its secular communication and implementation, and Saint Louis is a key figure in the building of the ruling ideology of the French modern state, as it is through him that Christian ideals were fully transplanted to the secular practice of rule and justice.  An examination of the narrative tradition concerning the saintly king confirms our argument. Starting from his virtue of humility, the biographers and hagiography writers constructed the image of a saintly king living and ruling according to the ″truth.″ Apart from his humility, Saint Louis was also said to be a man of great charity and mercy. His care for the poor was thought to guarantee that truth should not be distorted by greed in justice and it formed the essential part of his judicial policy. Under the pen of these writers, Saint Louis was also an honest king who only talked about truth and never told lies, even to his enemies. His personal virtues were thought to be extended to his practice of governance, and he was said to have governed truly, justly and saintly. The intimate connection of Saint Louis and truth is then to be found in his legislative and judicial activities. Saint Louis is constructed as an exemplary judge whose divine wisdom permitted him to surpass sheer legalism. Such construction should be understood in light of contemporary theological and legal discussions on the ways to obtain truth in justice. Apart from theoretical and intellectual developments, the age of Saint Louis witnessed major institutional developments that would lay the foundation of the judicial system of the Ancient Regime of France. It is under his reign that the activities of the Parliament of Paris were regularized, and that the king was able to extend his judicial power over major principalities. Also, under the influence of the Roman-canonical law, Saint Louis initiated systematic inquests in his realm and justified the responsibility of the king to supervise customs in the name of “truth”. He thus realized the union of the truth ideology with royal governance, and helped to lay down the moral and institutional framework of the French modern state.