The concept of popular consent occupies a central place in theories of political legitimacy, both in modern political philosophy and in contemporary Islamic political jurisprudence. Its significance lies in the fact that any account of legitimacy that invokes the people must clarify what consent means, what normative force it carries, and how it relates to political obligation and obedience. This article examines the place of popular consent in the political thought of John Locke and Muhammad Baqir al-Sadr, asking how consent functions in the establishment, actualization, and continuity of legitimacy, and how A. John Simmons’s critique of consent theory can illuminate the strengths and limitations of both models. Methodologically, the study is qualitative and library-based, employing a conceptual-comparative analysis. It first reconstructs Locke’s and al-Sadr’s accounts of consent within their respective theoretical contexts, and then evaluates their implications for legitimacy and political obligation through Simmons’s critical framework. The article argues that, in Locke’s theory, consent serves as the founding source of political power and the basis of the moral obligation to obey. Yet Locke’s reliance on tacit consent exposes his theory to serious difficulties concerning the authenticity, voluntariness, and measurability of consent. By contrast, al-Sadr does not treat popular consent as the primary source of legitimacy; rather, he regards it as a condition for the political actualization, continuity, and integrity of Islamic government. Consent in al-Sadr’s model is realized through participation, bayʿa, public oversight, and the “atmosphere of infallibility” (*jaww al-ʿiṣma*). The article concludes that politically valid consent must be real, institutionally traceable, and accompanied by effective possibilities of dissent, oversight, and restraint of power.