The current study examines the mediating role of adolescents' values in the relationship between religion and adolescents' self-reported positive and negative out-group and in-group attitudes. 1790 adolescents (among them 1102 Christians and 372 Muslims, mean age = 15,17 years) completed assessments of their religious denomination, individual values and perception of ingroups and out-groups. The self/other perception was represented by way of the constructs ethnocentrism and patriotism (perception of the in-group) on the one hand and the concepts discrimination, acceptance and tolerance (perception of out-groups) on the other. Referring on social identity theory, religion as a social categorisation was expected to have an impact on the self/other perception. Muslim and Christian adolescents were therefore assumed to differ in their in-group and out-group attitudes. T-tests yielded significant differences between Muslim and Christian adolescents, in line with the hypotheses, with regard to discrimination, acceptance and tolerance. Regression analyses and simple mediation models suggested that the values family and solidarity mediated the relationship between religion and adolescents' out-group attitudes of discrimination and acceptance. In addition, the impact of religion on the in-group attitude of patriotism was shown to be mediated by solidarity and family. Solidarity was also found to mediate the effect of religion on tolerance. The current study has important implications for (inter) religious education.