Ample research demonstrates that people are more prosocial toward ingroup than outgroup members, and that religious believers (e.g., Christians) tend to be more prosocial than non-believers (e.g., atheists), in economic games. However, we identify a condition under which ingroup biases in such games are attenuated, focusing on prosociality among atheists. Specifically, we argue that atheists (but not Christians) experience unique reputational concerns due to stereotypes that their group is immoral, which in turn affect their behavior toward outgroup partners. Across three studies, when participants in a Dictator Game believed their religious identity was known to their partner, atheists behaved impartially toward ingroup and outgroup partners, whereas Christians consistently demonstrated an ingroup bias. The effects of religious identity on allocations to the outgroup were partially mediated by concerns about being perceived negatively by others and were eliminated by telling participants that their religious identity would be kept anonymous.