Multiculturalism has been a relatively recent trend in Korea and Japan. Since the 1990s, both countries have been witnessing a rising influx of immigrants, the majority of whom were labor migrants and marriage migrants. Labor migrants have been accepted only as temporary residents. Contrary to this, marriage migrants have been seen as permanent residents, who need to integrate into the family and society. Both the Korean and Japanese governments have been providing them with relatively easy access to permanent residency and naturalization. Despite this effort, marriage migrants experience legal vulnerability, and they experience prejudice and discrimination with their families and society. This problem, even though highlighted by politicians and academics, has been persisting in society, and it has usually been researched as a policy problem or phenomenon that happens independently of its societal surroundings. However, because unequal treatment of migrants is related to social conditions as well as legal and policy issues, this study approaches the problem by examining marriage migrants' citizenship on three levels: legal, social, and identity. The study elaborates the causes behind their mistreatment through national identity and related nationalism. Both Korea and Japan possess an ethnic sense of belonging; they are perceived as ethnically homogeneous, monocultural nations. Therefore, the integration of ethnically different migrants presents a significant challenge. Ethnic nationalism in both countries manifests also in its subform, nouveau-riche nationalism. It is responsible for deep-seated beliefs in cultural superiority and evaluating people based on their country of origin and skin color. The study demonstrates the impact of ethnic nationalism on the legal level of citizenship in both countries and elaborates that this impact has been more restrictive in Japan than in Korea. Furthermore, nouveau-riche nationalism is shown to have a negative impact on the social and identity levels of citizenship in both counties, whereas in Japan social stigma and prejudices are shown to be more serious than in Korea.