Imam Musa Sadr is one of the most influential Muslim theorists and practitioners in the modern era. Therefore, it is essential to investigate his social ideas and their ontological foundations. In this paper, we studied the ontological foundations of Imam Musa Sadr’s social thought in his collection of speeches and writings using the document analysis method. Sadr considers humanity to have four aspects: an individual aspect that governs his freedom, a social aspect in relation to other humans, a community aspect as a part of the universe, and a divine aspect that governs the role of the divine caliphate of humanity in organizing the other aspects. From Sadr’s perspective, humanity is the only transformative force in existence and history, occupying the caliphate of God. According to him, the system of existence is created and the manifestation of God’s good attributes, living and sentient. As God's creation, the universe represents his attributes and is moving toward Him. Each component of the universe has a distinct function and direction. In addition, a stable system and the law of causality exist among them.Without God, there is no longer any time for the universe to exist. In addition, it is governed by lawfulness and intent. Sadr views love as the basis of existence, the family as the cell and substance of society, and social relations between families as its form. He views the formation of society as the result of material and spiritual interaction and exchange among individuals. Ummah is not a static entity, but the result of people’s unity in thought and mission, which leads to their unity and cohesion. Moreover, due to God’s unity, all creatures are equal in terms of existence. In addition to being relative, evil is the cause of human development and discretion. Sadr believes that the existence of the afterlife is essential for the establishment of justice. Also contemplates birth and death based on rules that, if broken, result in death.