This article examines the effect of blasphemy laws on Islamist terrorism in Muslim-majority countries. Although passed with the ostensibly noble purpose of defending religion, I argue that blasphemy laws encourage terrorism by creating a culture of vigilantism in which terrorists, claiming to be the defenders of Islam, attack those they believe are guilty of heresy. This study empirically tests this proposition, along with alternative hypotheses, using a time-series, cross-national negative binomial analysis of 51 Muslim-majority states from 1991–2013. It finds that states that enforce blasphemy laws are indeed statistically more likely to experience Islamist terrorist attacks than countries where such laws do not exist. The statistical analysis is supplemented with a brief case study of blasphemy laws and terrorism in Pakistan. The conclusion situates the findings in the context of policy.