The principle of non-maleficence, primum non nocere, has deep roots in the history of moral philosophy, being endorsed by John Stuart Mill, W. D. Ross, H. L. A. Hart, Karl Popper and Bernard Gert. And yet, this principle is virtually absent from current debates on social justice. This article suggests that non-maleficence is more than a moral principle; it is also a principle of social justice. Part I looks at the origins of non-maleficence as a principle of ethics, and medical ethics in particular. Part II introduces the idea of non-maleficence as a principle of social justice. Parts III and IV define the principle of justice as non-maleficence in terms of its scope and coherence, while Part V argues that the motivation of not doing harm makes this principle an alternative to two well-established paradigms in the literature on social justice: justice as mutual advantage (David Gauthier) and justice as impartiality (Brian Barry).