The issue of lethal autonomous weapons systems (laws) goes to the heart of the debate on new warfare technologies: States, international organizations, non-governmental organizations and civil society at large have long been discussing the acceptability of ‘autonomous killing’. The present contribution zooms in on the position held by the Holy See, exploring its content and the main arguments which support the call of a ban on such technology. Both diplomatic statements and doctrinal teachings will be tackled. Importantly, a solid argument for a prohibition of laws is based on the moral unacceptability of autonomous killing, which may assume also a legal standing through the so-called Martens Clause. The history and the actual content of the Clause will be analyzed in order to explore whether – and to what extent – it can be interpreted so as to offer a legal ground for rejecting laws. It will be argued that the Holy See is in a particularly fit position to advocate for a renewed appraisal of the Martens Clause that may help the pro-ban front to structure a more principled debate.