PROJECT SUMMARY Breast cancer is the most common cancer in younger women (≤ 50 years at time of diagnosis), who comprise approximately 19% of incident breast cancer cases. Improved survival after a breast cancer diagnosis has focused attention on the impact of the disease and its treatments on long-term outcomes in younger women. Studies have consistently shown that younger women have greater psychological and physical morbidity after breast cancer than older women and age-matched women with no cancer history, including elevated levels of depression. Depression has been linked to treatment non-adherence and predicts shorter recurrence-free and overall survival in women with breast cancer, highlighting its clinical relevance. Younger breast cancer survivors also report high levels of fatigue, insomnia, vasomotor symptoms, and cancer-related stress that may persist for years after diagnosis, causing significant impairment in quality of life. There is a critical need for effective, scalable interventions that can reduce depression and enhance long-term survivorship in this vulnerable group. To meet this need, we will conduct a phase III randomized clinical trial testing two new digital versions of a brief mindfulness intervention designed for younger breast cancer survivors. This study concept has been peer- reviewed and endorsed by the National Cancer Institute (NCI) and the NRG Oncology National Community Oncology Research Program (NCORP) which will implement the study protocol through its network of community and clinical sites, ensuring a broad and diverse group of participants. The Mindful Awareness Practices (MAPs) intervention has demonstrated efficacy in reducing depression and improving physical and psychological well- being in younger breast cancer survivors when delivered in person, in groups. We have developed and pilot tested two digital versions of the intervention, one delivered live online in groups over Zoom with an experienced instructor (MAPs live on