Abstract Taiwan as an island country is an immigrant society where interethnic marriages have been common. In the past centuries, it has not been unusual that people in Taiwan change their ethnic identities for various reasons. Its “four great ethnic groups” (sida zuqun)—the Hoklo, Hakka, Mainlanders, and aboriginal peoples—exist only as a social construction that arose in the 1990s in a specific political–cultural context. In 2005, a major government-sponsored research project, the Taiwan Biological Sample Bank—or Taiwan Bio-Bank (TBB)—was organized by a group of scientists and physicians. The purpose of the project is to collect genetic data from the “four great ethnic groups” of Taiwan in order to build a national database. This paper has three parts. First, I briefly review how the relationship between genetic attribution and the conceptualization of race/ethnicity has been addressed by social scientists and medical researchers since the mid twentieth century. Secondly, I analyze the history of biomedicine in Taiwan and the social and political context of the formation of the TBB project and its precursors. Thirdly, I examine the TBB’s theoretical and methodological assumptions about race/ethnicity in Taiwan. I point out that the rise of biomedicine since the 1990s has contributed to the “ethnicization of biomedicine,” in the sense that scientists have used dubious ethnic/racial categories with increasing frequency in practicing biomedicine.