Cross-national empirical research about the link between income inequality and population health produces conflicting conclusions. We address these mixed findings by examining the degree to which the income inequality and health relationship varies with economic development. We estimate fixed-effects models with different measures of income inequality and population health. Results suggest that development moderates the association between inequality and two measures of population health. Our findings produce two generalizations. First, we observe a global gradient in the relationship between income inequality and population health. Second, our results are consistent with income inequality as a proximate or conditional cause of lower population health. Income inequality has a 139.7% to 374.3% more harmful effect on health in poorer than richer countries and a significantly harmful effect in 2.1% to 53.3% of countries in our sample and 6.6% to 67.6% of the world’s population but no significantly harmful effect in richer countries.