Trust plays an essential role in social interactions. However, a number of empirical studies have indicated lower levels of trust toward foreigners in increasingly heterogeneous societies. What might explain outgroup discrimination in trust situations? While cultural sentiments and people's feelings of group-belonging are frequently evoked as potential explanatory factors, this paper argues that collectively irrational outgroup discrimination can also emerge on the basis of individually rational behavior. By means of an agent-based model (ABM), it is shown how the sole fact that individuals distinguish between group memberships when learning about trustworthiness of others leads to an anti-outgroup bias even if actual levels of trustworthiness are constant across groups. If further verified empirically, this finding might bear far-reaching implications for how group-based distrust should be countered.