We compared the relation between educational performance scores in the Programme for International Student Assessment (PISA) and Trends in International Mathematics and Science Study (TIMSS) on the one hand, and religiosity, as measured with the World Values Survey and the European Social Survey, on the other hand. We found that higher levels of religiosity (at national level) were associated with lower educational performance in science and mathematics (rs ranging from − 0.65 to − 0.74). One of the unique contributions of our data set is the ability to examine these trends by sex. Interestingly, even though women reported considerably higher levels of religiosity than men, this gap was not related to sex differences in educational performance. This latter finding constrains conclusions about the possible causal pathways between education, religiosity, and intelligence. Further, the mediating role of human development and time spent on religious education appear to account for the relation between education and religiosity. One possibility is that the relation between education and religiosity at the national level is related to overall levels of economic and human development, including investment in secular education.