Community-engaged research on environmental problems has reshaped researcher-participant relationships, academic-community interaction, and the role of community partners in human subjects protection and ethical oversight. We draw on our own and others’ research collaborations with environmental health and justice social movement organizations to discuss the ethical concerns that emerge in community-engaged research. In this paper we introduce the concept of reflexive research ethics: ethical guidelines and decision-making principles that depend on continual reflexivity concerning the relationships between researchers and participants. Seeing ethics in this way can help scientists conduct research that simultaneously achieves a high level of professional conduct and protects the rights, well-being, and autonomy of both researchers and the multiple publics affected by research. We highlight our research with community-based organizations in Massachusetts, California, and Alaska, and discuss the potential impacts of the community or social movement on the research process and the potential impacts of research on community or social movement goals. We conclude by discussing ways in which the ethical concerns that surface in community-engaged research have led to advances in ethical research practices. This type of work raises ethical questions whose answers are broadly relevant for social movement, environmental, and public health scholars.