Iranianate political thought, as one of the most enduring intellectual traditions in the Iranian civilizational sphere, has historically shaped conceptions of sovereignty, legitimacy, and the state–society relationship. Throughout various historical periods, rulers relied on this tradition to consolidate the ideological underpinnings of their authority, reinforce political cohesion, and legitimize mechanisms of governance. This study examines the role of Iranianate political thought in shaping the foundations of legitimacy and the subsequent decline of the Second Pahlavi regime. Employing an analytical-historical approach grounded in conceptual-comparative analysis, the research draws upon historical documents, library sources, and content analysis of ideological symbols and policy discourses of the Pahlavi era. The guiding question asks how Iranianate political thought influenced the legitimacy of Mohammad Reza Shah’s rule and what role this tradition played in the regime’s decline. The central hypothesis argues that the Shah sought to enhance his political legitimacy by invoking themes of ancient symbolism, Iranian nationalism, religious traditionalism, and the Iran-Shahr royal ideology. Nevertheless, the predominance of imitative modernisation, the neglect of social justice, the weakening of religious institutions, and insufficient attention to cultural sensitivities widened the state–society gap. The findings reveal that, although the regime employed Iranianate elements to construct symbolic legitimacy, the deeper doctrinal components of this tradition were never institutionalised within the Pahlavi political structure. The resulting divide between authentic Iranianate principles and their superficial representation, combined with externally dependent modernisation and an overreliance on coercive instruments, ultimately undermined political legitimacy and contributed to the regime’s collapse.