This article considers Hobbes’ contribution to the development of constitutionalist thought by contextualizing his treatment of the concepts of treason and fundamental law in De cive (1642, 2nd ed. 1647) and Leviathan (1651). While in Leviathan he adopts the controversial conception of treason as a violation of fundamental law that had been employed to convict Charles I of high treason in 1649, he draws on the original meaning of the term “fundamental law”, as outlined in the most influential early analysis of Innocent Gentillet, to deny that fundamental laws can constrain the rights and powers of the sovereign. He bolsters this position by treating fundamental law as natural, not civil, law. While citizens commit treason when they violate the original covenant that establishes the sovereign, citizens cannot appeal to a human court for violations of fundamental law by the sovereign (who must render account for violations of natural law only to God). Hobbes’ ingenious reconceptualization of fundamental law, hence, shows that, when understood correctly, the theory of treason embraced by parliamentarians could never support the violent resistance against, and overthrow of, a monarch like Charles I.