This entry assesses the concept of rationalism, which is widely understood across disciplines, from the viewpoint of Islamic economics. In philosophy, rationalism is influenced by discussions over the origin of knowledge, whether it stems from experience or reason, whereas in economics, rationalism emphasizes individual economic behavior. Given that economics is a discipline grounded in human decisions and behaviors, definitions and assumptions regarding individuals are crucial to economic rationalism. In traditional economics, the rational individual is referred to as “homo-economicus,” a being that aims to achieve maximum benefit with minimal effort, possesses complete information to optimize its interests, and makes decisions based on rationality. Homo-economicus operates autonomously from psychological, normative, or moral influences in decision-making and structures all actions to optimize personal interests. This paradigm is grounded in instrumental rationalism, which is the foundation of classical and neoclassical economic theories. Instrumental rationalism relies on the identification of the most suitable means to achieve an objective and posits that reason should optimally determine this means-end relationship in individual acts to fulfill their motives. In contrast to the homo-economicus model, Islamic economics presents a normative model of the individual, termed “Homo Islamicus,” whose economic choices are influenced by Islamic moral and religious principles. Islamic economics posits that individuals should pursue not only self-interest and collective welfare. The comprehension of rationalism in Islamic economics is founded on substantive rationality, a form of rationality directed by religious and moral norms that relies on value-oriented reasoning rather than merely means-ends analysis.