Agentic self-enhancement consists of self-protective and self-advancing tendencies that can lead to aggression, especially when challenged. Because self-enhancers often endorse aggression to defend or enhance the self-concept, religious self-enhancement should lead to endorsing aggression to defend or enhance one’s religion. We recruited three samples ( N = 969) from Mechanical Turk ( n = 409), Iran ( n = 351), and the U.S.–Mexico border region ( n = 209). We found that religious (but not secular) self-enhancement in the form of religious overclaiming predicted support for, and willingness to engage in, religious aggression. In contrast, accuracy in religious knowledge had mostly negative associations with aggression-relevant outcomes. These results emerged across two separate religions (Christianity and Islam) and across three different cultures (the United States, Iran, and the U.S.–Mexico border region). Thus, religious overclaiming is a promising new direction for studying support for religious aggression and identifying those who may become aggressive in the name of God.