Objective: We test whether prejudice can influence lay attributions of mental illness to perpetrators of violence. Specifically, we examine whether people with negative attitudes towards Muslims perceive Muslim mass shooters as less mentally ill than non-Muslim shooters. Method: Study 1 compares attributions of mental illness to Muslim and non-Muslim perpetrators of recent mass shootings. Studies 2 and 3 experimentally test whether a mass shooter described in a news article is seen as less mentally ill when described as being a Muslim, compared to when described as a Christian (Study 2) and to when religion is not mentioned (Study 3). Study 4 tests whether a Muslim shooter is seen as less mentally ill than a Christian shooter, even when both shooters have symptoms of mental illness. Results: In all studies, Muslim shooters were seen as less mentally ill than non-Muslim shooters, but only by those with negative views towards Muslims. Conclusion: Those with anti-Muslim prejudices perceive Muslim mass shooters as less mentally ill, likely to maintain culpability and fit narratives about terrorism. This may reinforce anti-Muslim attitudes by leading those with anti-Muslim prejudice to overestimate the amount of violence inspired by groups like ISIS relative to extremist groups from other ideologies.