Objective Prevention of eating disorders (EDs) is of high importance. However, digital programs with human moderation are unlikely to be disseminated widely. The aim of this study was to test whether a chatbot (i.e., computer program simulating human conversation) would significantly reduce ED risk factors (i.e., weight/shape concerns, thin-ideal internalization) in women at high risk for an ED, compared to waitlist control, as well as whether it would significantly reduce overall ED psychopathology, depression, and anxiety and prevent ED onset. Method Women who screened as high risk for an ED were randomized (N = 700) to (1) chatbot based on the StudentBodies (c) program; or (2) waitlist control. Participants were followed for 6 months. Results For weight/shape concerns, there was a significantly greater reduction in intervention versus control at 3- (d = -0.20; p = .03) and 6-m-follow-up (d = -0.19; p = .04). There were no differences in change in thin-ideal internalization. The intervention was associated with significantly greater reductions than control in overall ED psychopathology at 3- (d = -0.29; p = .003) but not 6-month follow-up. There were no differences in change in depression or anxiety. The odds of remaining nonclinical for EDs were significantly higher in intervention versus control at both 3- (OR = 2.37, 95% CI [1.37, 4.11]) and 6-month follow-ups (OR = 2.13, 95% CI [1.26, 3.59]). Discussion Findings provide support for the use of a chatbot-based EDs prevention program in reducing weight/shape concerns through 6-month follow-up, as well as in reducing overall ED psychopathology, at least in the shorter-term. Results also suggest the intervention may reduce ED onset. Public Significance We found that a chatbot, or a computer program simulating human conversation, based on an established, cognitive-behavioral therapy-based eating disorders prevention program, was successful in reducing womens concerns about weight and shape through 6-month follow-up and that it may actually reduce eating disorder onset. These findings are important because this intervention, which uses a rather simple text-based approach, can easily be disseminated in order to prevent these deadly illnesses. Trial registration: OSF Registries;