This study delves into the pivotal role of Islam in the foreign policy of the Islamic Republic of Iran, examining its profound influence on domestic and international affairs. The revolution of 1979, occurring amid discussions of secularism, marked a significant turning point, as it reintroduced a purified form of Islam as a central ideology. The Islamic Republic of Iran stands out as a unique case within the modern international order, being the first and perhaps the only religious revolution in this context. It not only imposed its revolutionary ideals on the prevailing international order but also introduced the principles of religion into the international regime. This investigation explores the multifaceted role of Islam in the realm of international relations theory, highlighting its significance as a key variable in shaping Iran's foreign policy. It also examines how the dominant theories in international relations can be used to analyze Iran's foreign policy, considering the complex interplay between Islamic principles and international realities. Unlike other Muslim countries, Iran's interpretation of Islam is deeply ingrained in its political authority and daily life, making it a distinct entity in the international arena. The study further underscores the unique nature of the Iranian revolution, which introduced the concept of velayat-e faqih (Guardianship of Islamic Jurists) based on Islamic law and reshaped the country's governance system, both domestically and in its foreign affairs.