ABSTRACT: BACKGROUND:Younger breast cancer survivors consistently report a greater impact of their cancer experience on quality of life compared with older survivors, including higher rates of body image disturbances, sexual dysfunction, and fatigue. One potential strategy to improve quality of life is through physical activity, but this has been understudied in younger breast cancer survivors, who often decrease their activity during and after cancer treatment. OBJECTIVE:The aim of this study is to explore the feasibility and acceptability of a technology-based, remotely delivered, peer-led physical activity intervention for younger breast cancer survivors. We will also assess the preliminary impact of the intervention on changes in physical activity and multiple aspects of quality of life. METHODS:This study is a community-academic partnership between University of California, San Diego and Haus of Volta, a nonprofit organization that promotes positive self-image in younger breast cancer survivors. This ongoing pilot study aims to recruit 30 younger breast cancer survivors across the United States (<55 years old, >6 months post primary cancer treatment, self-report <60 min of moderate-to-vigorous-intensity physical activity [MVPA]) into a 3-month peer-delivered, fully remote exercise program. Participants will complete 6 biweekly video chat sessions with a trained peer mentor, a fellow younger breast cancer survivor. Participants will receive a Fitbit Charge 3; weekly feedback on Fitbit data from their peer mentor; and access to a private, in-app Fitbit Community to provide and receive support from other participants and all peer mentors. At baseline, 3 months, and 6 months, participants will complete quality of life questionnaires, and MVPA will be measured using the ActiGraph accelerometer. Feasibility and acceptability will be explored through a mixed methods approach (ie, quantitative questionnaires and qualitative interviews). Intervention delivery and adaptations by peer mentors will be tracked through peer mentor self-evaluations and reflections, review of video-recorded mentoring sessions, and monthly templated reflections by the research team. RESULTS:Recruitment began in September 2019. As of February 2020, the physical activity intervention is ongoing. Final measures are expected to occur in summer 2020. CONCLUSIONS:This study explores the potential for physical activity to improve sexual function, body image, and fatigue, key quality of life issues in younger breast cancer survivors. Using peer mentors extends our reach into the young survivor community. The detailed process evaluation of intervention delivery and adaptations by mentors could inform a future hybrid-effectiveness implementation trial. Finally, remote delivery with commercially available technology could promote broader dissemination. TRIAL REGISTRATION:ClinicalTrials.gov NCT04064892; https://clinicaltrials.gov/ct2/show/NCT04064892. INTERNATIONAL REGISTERED REPORT IDENTIFIER (IRRID):DERR1-10.2196/18420.