Although there are many studies on John Calvin’s teaching on natural law, the relation between natural law and Roman law has received relatively less attention. This essay examines the relation between natural law and Roman law in Calvin’s exegetical writing on the Mosaic law. I argue that Calvin regarded Roman law as an exemplary, albeit imperfect, witness to the natural law, and he used Roman law to aid in his interpretation of the Mosaic law. Since he assumed that Roman law embodies principles of natural law, Calvin drew on Roman law as an aid in order to distinguish natural from positive law within the Mosaic law. He also broadened the scope of commandments in the second table of the Decalogue by comparison with natural and Roman law. Yet although Calvin drew many continuities between Mosaic and Roman laws, he remained critical of the Roman system due to various failings in comparison with Scripture and principles of natural law.