This Article analyzes how individual and local attempts to address low fertility rates in Korea and Japan have prompted unprecedented reforms in monocultural nationality laws. Korea and Japan confront rapidly declining working-age population projections; yet, they have prohibited the immigration of unskilled workers, until recently in Korea's case, on the claim that their admission would threaten social cohesion. Over the past two decades, both countries have made only incremental reforms to their immigration policies that fall short of alleviating labor shortages and the fiscal burdens of maintaining a large elderly population. Instead, prompted by the growth of so-called multicultural families in each country, the most significant reforms have appeared in nationality laws: Korea's introduction of dual nationality in 2010 and Japan's 2008 reform to grant Japanese nationality to children born out of wedlock to a Japanese father and a foreign mother. While neither country has introduced birthright citizenship to their nationality laws, both sets of reforms target ethnically heterogeneous immigrant women, their bicultural children, or both. This Article seeks to understand how recent reforms to nationality laws in Korea and Japan have challenged long-held conceptions of blood-based belonging and membership.