Background Citizen science and models for public participation in health research share normative ideals of participation, inclusion, and public and patient engagement. Academic researchers collaborate in research with members of the public involved in an issue, maximizing all involved assets, competencies, and knowledge. In citizen science new ethical issues arise, such as who decides, who participates, who is excluded, what it means to share power equally, or whose knowledge counts. This article aims to present an ethics framework that offers a (...) lens of understanding and heuristic guidelines to deal with ethical issues in citizen science. -/- Methods We conducted seven case studies between 2015 and 2021 to attune and validate the ethics framework for the context of citizen science. The cases related to studies with older adults, people with a psychiatric vulnerability, people dependent on community care, people who are unemployed or living in poverty or both, and young adults with respiratory disease. -/- Results Ethics in citizen science reaches beyond the ethical issues in traditional biomedical and health research. It entails more than following procedures about informed consent and privacy and submitting a proposal to a Medical Research Ethics Committee. Ethics in citizen science relates to everyday ethical issues during the study, including relational and moral complexities concerning collaboration, sharing power, and democratic decision-making. Dealing with these issues requires ethics work of researchers. This entails seeing ethically salient issues and reflecting on everyday ethical issues. Ethics work consists of seven features: framing work, role work, emotion work, identity work, reason work, relationship work, and performance work. All are relevant for researchers in citizen science. -/- Conclusions Ethical issues in citizen science often relate to power differentials, partnership, and collaboration between academics and non-academics. The ethics framework prepares researchers for the work needed in citizen science to act responsibly and offers a heuristic guide to reflect on ethics. Reflection on ethics is a pathway towards ethical citizen science, especially if researchers collaboratively reflect in partnership with non-academics who are subject to the moral issue. (shrink)