The moral, as opposed to legal, justification for the preventive detention of terrorists is the topic of this article, and, in particular, for the preventive detention of members of extremist Islamist terrorist organizations, such as Islamic State (IS) and Al Qaeda (AQ). The article argues that preventive detention of terrorists is morally justified under certain circumstances. Its argument for preventive detention of terrorists is analogous to that used to detain enemy combatants as prisoners of war. However, rather than relying on the possession of the properties definitive of the legal status of a combatant, it relies on demonstrable possession of constitutive features of functionally integrated membership of a terrorist organization. Membership in this sense of a terrorist organization creates the presumption of a standing intention to commit murder, or at least to assist others to do so, in the service of the organization’s political ends.