The relationship between religion and national identity is one of the fundamental challenges of modern Iranian thought. This study, focusing on the thought of Mīrzā Yūsuf Khān Mustashār al-Dawla, examines the role of Sharia in redefining national identity within the discourse of constitutionalism. The main question is how Mustashār al-Dawla utilized intra-religious capacities to strengthen national identity and what kind of relationship between religion and nationality crystallized in his thought. Employing a descriptive-analytical method and Quentin Skinner's hermeneutic approach—analyzing language, authorial intention, and social context—the findings show that by distinguishing between custom ('urf) and religious law (shar'), Mustashār al-Dawla elevated Sharia from a purely jurisprudential system to a moral-social one, legitimizing modern concepts such as law, justice, equality, and liberty within a religious framework. He identified these four pillars as the core elements of modern national identity, and by redefining them in the language of Sharia, he paved the way for the formation of a new national identity in Iran that remained faithful to religious tradition while aligning with the demands of modernity.