What explains the origins of nationalism? In ihis article, we trace the origins of an important early 20th century nationalist movement to Christian missionary activity. A growing literature credits missionaries with spreading schooling and democracy. Yet missionary activity often led to a political backlash and to anti-foreign, nationalist mobilization. Drawing on evidence from China, we show how missionary activity sparked nationalist mobilization in the early 1900s. We gather new data on early nationalist secret societies, missionary activity, and anti-missionary violence. Qualitative and quantitative evidence show how missionaries threatened the political power of local elites, who responded by mobilizing violent anti-foreign protests and participating in nationalist political societies. The findings challenge the idea that Christian missionaries influenced long-run political development primarily because they spread schooling and literacy. Instead, we show that missionaries also sparked early nationalist mobilization.