As datafication progressively invades all spheres of contemporary society, citizens grow increasingly aware of the critical role of information as the new fabric of social life. This awareness triggers new forms of civic engagement and political action that we term “data activism”. Data activism indicates the range of sociotechnical practices that interrogate the fundamental paradigm shift brought about by datafication. Combining Science and Technology Studies with Social Movement Studies, this theoretical article offers a foretaste of a research agenda on data activism. It foregrounds democratic agency vis-à-vis datafication, and unites under the same label ways of affirmative engagement with data (“proactive data activism”, e. g. databased advocacy) and tactics of resistance to massive data collection (“reactive data activism”, e. g. encryption practices), understood as a continuum along which activists position and reposition themselves and their tactics. The article argues that data activism supports the emergence of novel epistemic cultures within the realm of civil society, making sense of data as a way of knowing the world and turning it into a point of intervention and generation of data countercultures. It offers the notion of data activism as a heuristic tool for the study of new forms of political participation and civil engagement in the age of datafication, and explores data activism as an evolving theoretical construct susceptible to contestation and revision.