Scholars often have used the group threat thesis to explain why punitiveness varies across places. This research regularly has found that punitiveness is harsher in places with a larger minority population. Yet researchers only have had a rudimentary grasp of why this is the case. Moreover, most prior research has focused only on the United States, giving us little knowledge of whether the group threat thesis is a viable explanation of cross-national differences in punitiveness. In the current study, we postulate that the relative size of the out-group population affects punitiveness indirectly, via its impact on individual intolerance toward ethnic out-groups. We test this thesis cross-nationally with data from individuals residing in 27 European countries. Our findings are consistent with the argument that greater racial/ethnic diversity at the country level affects individuals’ attitudes toward minority out-groups, which in turn increases their support for severely punishing criminal offenders.