Contemporary legal strategies for obesity raise troubling questions regarding individual liberty and the legitimate scope of public health authority. This article argues that the predominant approach to assessing public health legitimacy—John Stuart Mill’s “harm principle”—may be unsuitable for evaluating the legitimacy of legal strategies for obesity. The article proposes an alternative test for assessing the legitimate scope of public health authority: John Rawls’s liberal principle of legitimacy. It outlines how Rawls’s principle would evaluate obesity policies, and contrasts this evaluation to that of Mill. The alternative test avoids some of the limitations of the Millian approach, and may offer an improved mechanism for assessing the liberty effects of policies for obesity and other public health activities.