Courage is an important moral virtue for both Aristotle and Aquinas. For Aristotle, courage is a virtue that belongs to warriors who are ready for a noble death on the battlefield. As a Christian theologian as well as an Aristotelian expert, Aquinas aims to give this Aristotelian moral virtue a fully theological expression. This paper analyzes the differences between Aquinas’s conception of courage and Aristotle’s, as well as explores Aquinas’s transformation of Aristotelian courage through a three part process. Firstly, based on Aristotle’s paradigm of courageous warriors in battle, Aquinas extends the scope of “battle” from the military sense to a broader one. By doing so, Aquinas expands the range of application of courage. Secondly, Aquinas explicitly defines endurance as the chief act of courage based on the reason that endurance is more difficult than aggression, thereby shifting our attention from the attack aspect of courage to the endurance aspect. Finally, Aquinas defines the principal act of perfect courage as martyrdom thereby pointing to Christ, who was the perfect martyr, as the paradigm of a courageous person. The result of this transformation is a successful theological virtue of courage.