Purpose Breast cancer treatment adverse effects result in one in three survivors experiencing body image–related distress (BID) that negatively impacts on a woman’s ability to recover after cancer and into survivorship. My Changed Body (MyCB) is a Web-based psychological intervention to alleviate BID and improve body appreciation in survivors of breast cancer (BCSs) through a single-session, self-compassion focused writing activity. This randomized controlled trial evaluated the impact of MyCB on BID and body appreciation in BCSs. The moderating effect of lymphedema status (affected or unaffected) and appearance investment (self-importance placed on personal appearance) and the mediating effect of self-compassion were evaluated. Patients and Methods Women (disease-free stage I to III BCSs who had experienced at least one negative event related to bodily changes after breast cancer) were randomly assigned to MyCB (n = 149) or an expressive writing control arm (n = 155). Primary outcomes were reduction in BID and improvement in body appreciation 1 week after intervention. Secondary outcomes included psychological distress (depression and anxiety) and self-compassion. Follow-up assessments occurred 1 week, 1 month, and 3 months after writing. Results Compliance with the MyCB intervention was 88%, and attrition was 9.2%. Intent-to-treat linear mixed models indicated that participants who received MyCB reported significantly less BID ( P = .035) and greater body appreciation ( P = .004) and self-compassion ( P < .001) than expressive writing participants. Intervention effects on BID were moderated by lymphedema status ( P = .007) and appearance investment ( P = .042). Self-compassion mediated effects on both primary outcomes. Therapeutic effects were maintained at 1 month (BID and body appreciation) and 3 months (body appreciation) after intervention. Significant reductions in psychological distress (1-month depression, P = .001; 1-week and 1-month anxiety, P = .007) were evident for MyCB participants with lymphedema. Conclusion This study supports the efficacy of MyCB for reducing BID and enhancing body appreciation among BCSs.