Protected areas are designed to conserve ecosystems and their services, but the restrictions they impose create the potential for unintended consequences. For instance, poverty advocates have long voiced concerns that protected areas might exacerbate poverty in surrounding communities. Here we examine another potential unintended consequence of protected areas: illegal activities. We use data from Colombia to estimate the impact that protected areas had on violence perpetrated by guerrilla groups. We find protected areas that were established prior to 2002 significantly increased the number of guerrilla attacks in affected municipalities during the surge of violence in the mid-2000s. Our results are robust to the choice of estimator and numerous additional tests. We find evidence that guerrillas were using protected areas as havens to conduct their operations and that our impact estimates are largely driven by protection in the most rural areas.