Aim <p>Young women with psychiatric illness have an increased susceptibility for eating disorders (ED); however, no prevention initiatives have been tested for this group. The Body Project ED prevention program has produced promising effect on ED symptoms, body satisfaction, and future ED onset; however, no studies have investigated this specific vulnerable group. In SMILE Body Project, we adapted the Body Project manual to a psychiatric population. In this protocol paper, we describe the randomized controlled trial (RCT) to evaluate the efficacy of the SMILE Body Project.</p> Methods <p>This is an ongoing RCT including 300 young women between 15 and 30&#xa0;years with a psychiatric illness and subjective body dissatisfaction, starting May 2025. Participants are randomized to either online Body Project groups or an active control arm based on expressive writing exercises. The primary aim is to test if SMILE Body Project significantly reduces the incidence of ED in women with psychiatric illness after 2&#xa0;years compared to controls. The secondary aim is to explore changes in ED symptoms, psychiatric symptoms, QoL, and predictors such as psychiatric symptoms and time spent on social media.</p> Results <p>We expect that SMILE Body Project will effectively reduce the incidence of ED diagnosis compared to controls. In addition, we anticipate new knowledge on how time spent on social media affects body image in women with psychiatric illness.</p> Conclusions <p>If SMILE Body Project proves efficacious, it has potential for implementation at low cost and may contribute to broad prevention of future ED onset among young women with psychiatric illness.</p> Clinical Trials. gov Identifier <p>NCT06893627</p>