This article demonstrates a method to measure the extent and variation of ethnoracial disproportion in world prison populations. Using a novel data set covering eighteen democracies for the year 2016, this method shows that conspicuous ethnoracial disproportion in prisons is pervasive in democracies for which data is available. Socioeconomically marginalized ethnic and racial groups are overrepresented in the prisons of every case. Further, this pattern extends to countries known for penal progressivism, which otherwise are regarded as models of fair and rational criminal justice implementation. Correlation between prison disproportion and national socioeconomic and criminal justice characteristics suggests the complexity of the issue. A selection of single-case analyses drawn from the sample describe prison disproportion emerging under significantly varied national conditions. I discuss causal hypotheses for why ethnoracial disproportion would occur in such differing places.