A debate exists about the economic and cultural-based drivers of support for populism. In this paper, we argue that economic concerns matter, but they are realised through the relative gains and losses of social groups. Using new survey items in a large representative survey in Britain, we show that citizens’ economic assessments of the ethnic minority out-group - in relation to the group’s 12 months ago and to assessments of the economic conditions of the white British in-group - are a predictor of support for Brexit. Results, which are robust to prior referendum vote, immigration attitudes, and cultural sentiment, extend across income groups and national identity strength. Extending the analysis to a comparison of geographic in- and out-groups between local communities and London lends additional support to our argument. The implications of relative group-based economics are important for understanding Brexit and the economic sources of support for populism more broadly.