While there is a renewed literature connecting internal migration to various issues related to structural transformation such as urban labor and housing markets, the relationship between internal migration and demographic transition is much under-studied despite its importance in the process of economic development. Our article fills this knowledge gap. By constructing a simple dynamic framework in which fertility and rural-urban migration decisions are both determined, we show that more-rapid urban productivity advancement can lead to a positive relationship between migration and fertility. Using cross-country data analysis, we support our theory, establishing that both migration and fertility rates are higher in less-developed countries than in advanced economies. Our results imply that policies that may help reduce the cost of urban living or enhance urban benefits would be useful for productive structural transformation.