This article has two principal objectives: (1) to study the behavioral dimensions of Muslim prisoners which predict their Islamist radicalism and (2) to study whether the behavior manifested by them is higher in prisons with a greater concentration of Muslims and a higher presence of prisoners convicted for Islamist terrorism than in prisons with fewer Muslims and no convicted Islamist terrorists. We conclude that some Spanish prisons may provide favorable social environments for jihadist radicalism and that the questionnaire utilized is a useful diagnostic tool for evaluating the magnitude of this phenomenon.