In this essay we argue that Thomas Aquinas’s interaction with Gratian’s Decretum constitutes a provocative yet relatively unexplored avenue of study. As a sample of this thesis, we analyze two passages where Aquinas explicitly refers to Gratian directly and the juridical tradition indirectly. The first passage pertains to Aquinas’s interpretation of Gratian’s definition of natural law in the Prima Secundae of the Summa Theologiae, but leads us to move to other places in Thomas’s tract on the virtue of justice and his commentary on Peter Lombard’s Sententiae. The second exemplification is found in Aquinas’s tract of the sacrament of penance in the closing pages of the Summa. Both instances are quick glimpses of a vastly larger intellectual domain awaiting exploration.