This article examines the doctrine of tyrannicide in John of Salisbury's mid-twelfth century political treatise, the Policraticus, in light of recent scholarly skepticism that John never meant to advocate a theoretical defense of slaying the tyrant. It is argued that John's conception of tyrannicide in fact possesses a philosophical foundation derived from his idea of the state as a political organism in which all the members cooperate actively in the realization of the common utility and justice. When the ruler of this body politic behaves tyrannically, failing to perform his characteristic responsibilities, the other limbs and organs are bound by their duty to the public welfare and God to correct and, ultimately, to slay the tyrant. John illustrates this position by reference to the many historical and scriptural instances of tyrants who have legitimately been killed. Thus, John not only proposes a theory of tyrannicide, but also roots it in a strong positive obligation to raise the sword against tyrannical rulers in the name of public benefit and justice.