Background: Eating disorders are prevalent, often have chronic courses and relapses are frequent even after effective treatment approaches. Therefore, prevention is decisive; however, many of the current prevention programs are resource intensive. Internet-based interventions can represent cost-effective and low threshold alternatives but only few approaches have so far been investigated.Objective: The aim of this study was to evaluate the efficacy of an internet-based unaccompanied preventive intervention.Material and methods: The intervention was newly developed based on behavior therapeutic techniques and piloted in a group of 200 students using a randomized waiting list control group design. Data on eating disorder-specific pathology (eating disorder examination questionnaire, EDE-Q), self-esteem (Rosenberg self-esteem scale, RSES), and well-being (World Health Organization-five well-being index, WHO-5) were collected before and after the intervention or the waiting period. Data were evaluated based on variance analysis.Results: A total of 43% of participants completed the intervention. Self-esteem increases were stronger in the intervention group in comparison to the waiting control group with large effect sizes (ηp2 = 0.33). There were no significant differences between the groups for the other variables.Conclusion: Unaccompanied online self-help appears to provide a promising approach for improving self-esteem thus contributing to the prevention of eating disorders. Investigations in larger and more heterogeneous groups are necessary in the future to identify possibly present smaller preventive effects .