The Ash‘arī school is one of the most influential currents in the history of Islamic theology. In response to the excessive rationalism of the Mu‘tazila and the influence of Greek philosophy, it offered a distinctive approach to the function of reason in understanding doctrinal matters. Nevertheless, the historical trajectory of this school shows that "rationalism" in Ash‘arī thought is not a fixed phenomenon but rather a multi-stage process that, from the time of Abū al-Ḥasan al-Ash‘arī to later theologians, underwent structural and epistemological evolution. The necessity of this research arises from the fact that most existing studies treat Ash‘arī rationalism as a monolithic whole, seldom analyzing its developmental process. In contrast, a careful examination reveals that the Ash‘arīs, in different phases, moved from al-Ash‘arī’s minimal rationalism to al-Bāqillānī’s systematic rationalism, and then to al-Juwaynī’s philosophical-theological rationalism. The main objective of this research is to explain the evolution of rationalism in the Ash‘arī school, focusing on three pivotal theologians—Abū al-Ḥasan al-Ash‘arī, al-Bāqillānī, and al-Juwaynī. First, the principal works of each theologian are subjected to content analysis. Then, to uncover the lines of development, a systematic comparison is made of their argumentative methods, rational positions, and epistemological structures. Furthermore, the historical, cultural, and philosophical contexts influencing this evolution are examined. The findings show that Ash‘arī rationalism followed a dynamic trajectory, explainable in three stages: limited rationalism, instrumental rationalism, and structured rationalism, thereby transforming from a cautious current regarding reason into a rational and philosophical theological school.