Although there have been many empirical studies of terrorism within the psychology literature, relatively few studies have theorized terrorism from the perspective of collective action theory. The present study aimed to understand factors that predict support for Islamist terrorism by using the Encapsulate Model of Social Identity in Collective Action (EMSICA) perspective. To extend previous studies, we added perception of threat and intergroup contact to the model. A correlational study using 66 terrorism detainees in Indonesia as participants found solid support for our expanded EMSICA. The models extending EMSICA with perception of threat and quantity of contact as predictors had improved fit indices and explained more variance in the dependent variable support for Islamist terrorism, as compared to the standard model. Social identity had a significant direct effect on support for Islamist terrorism and mediated the effect of intergroup contact, perception of threat, perceived injustice, and group efficacy on support for Islamist terrorism. © 2019 Asian Association of Social Psychology and John Wiley & Sons Australia, Ltd