Research into the field of citizen social science remains rather limited. Nevertheless, citizen groups emerge as co-researchers and co-designers in many research and innovation activities. The EU-funded CoAct project proposes a fundamentally new approach to tackling social global concerns related to mental health care, youth employment, environmental justice and gender equality: by engaging vulnerable citizens acting as co-researchers. The project will run in Barcelona, Vienna, Berlin and eastern Europe, as well as Buenos Aires. It will be implemented by a consortium of research institutions, NGOs and global networks of open science and open data activism. The project’s goal is identify and develop a general model for citizen social science that engages citizen bodies concerned with specific social issues in co-research.