In June 2013, the American Medical Association (AMA) made the highly controversial decision to designate obesity a disease. Proponents predicted the decision would lead to reduced weight-related stigma, whereas opponents predicted designating a third of the population as “diseased” would exacerbate stigma. To determine the effects of defining obesity as a disease on explicit and implicit weight-biased attitudes and explicit weight-biased beliefs, female undergraduate students (N = 146) were randomly assigned to one of two groups: disease or lifestyle. Participants in the disease group (n = 71) were asked to read an article describing obesity as a disease caused by biology and genes; participants in the lifestyle group (n = 75) read an article describing obesity as the result of personal choices, including over-consumption of food and inactivity. Explicit weight-biased attitudes and beliefs were measured pre- and post-exposure to the article. Change in beliefs about the controllability of weight was examined as a potential meditator of the relationship between group and explicit weight-biased attitudes; and body mass index (BMI), health orientation, and fitness orientation were examined as potential moderators. Results revealed a significant interaction between group and time on weight-biased beliefs. Participants in the disease group exhibited stronger beliefs that obesity is outside a person’s control from pre- to post-exposure, whereas participants in the lifestyle group exhibited a weakening in these beliefs over the same time period. Contrary to hypotheses, this change in beliefs about the controllability of weight did not extend to weight-biased attitudes.