It is a commonplace in the history of political theory to mark John of Salisbury as the first medieval defender of the so-called theory of tyrannicide according to which it is allowed and properly rightful to kill a tyrant. Yet commonplaces run the risk of platitudes: they need control and regular revision; from time to time the foundations of a platform have to be revisited in order to inspect their bearing-power. That is the intention of this communication, the first section of which will investigate some traditional arguments for the existence of such a theory; the second section will replace the problematic issue in the context of John’s works, especially of his main work, the Policraticus.