The aim of this essay is to analyze the treatment of the theory of natural slavery in Francisco de Vitoria’s through his reception of Aquino’s treatise on justice and the Aristotelian theory of natural slavery itself described in book I of Politics. In order to understand how Vitoria distances himself from Aquinas, by considering the ius gentium as a positive right, I analyze Victoria’s comments on the Summa Theologica. Finally, I consider Question 1 of the relectio De Indis, where Victoria directly criticizes the idea of natural slavery.