We study how the design of subnational political borders shapes the ethnicization of national politics. We introduce novel measures that approximate the extent to which political boundaries follow a country's ethnic geography. We then analyze the effects of the 2010 constitutional reform in Kenya, which divided the country's eight provinces into 47 counties and devolved power to these new counties. Using difference-indifference designs, we find that ethnofederal reforms, reforms that increase the alignment between political divisions and ethnic geography, reduce ethnic voting in national elections. A key channel is that ethnofederalism "tranquilizes" locally powerful groups that control the provision of local public goods. Finally, we evaluate the counterfactual effects of alternative boundary proposals on ethnic voting in Kenya.