Introduction: This pilot study aimed to investigate the effects of using an app-based certified medical product named PINK! on breast cancer patients and survivors. The objectives were to measure psychological distress, physical activity, and therapy-related fatigue of patients using PINK! to identify trends and develop a study design for a subsequent multicentric proof of efficacy RCT. Materials and Methods: PINK! offers individualized, evidence-based therapy and side-effect management, mindfulness-based stress reduction, nutritional and psychological education, physical activity tracking, and motivational exercises to implement lifestyle changes sustainably in daily routine. A prospective, intraindividual RCT was performed with n = 60 patients in 2021 at Comprehensive Cancer Center Munich. Patients with BC were included independent of the stage of diseases. The intervention group got access to PINK! over 12 weeks. Control group served as a waiting-list comparison to “standard of care.” Results: Primary efficacy variable analysis revealed a relative average decrease of 32.9% in psychological distress, which corresponds to a statistically significant reduction ( p < 0.001) within 12 weeks compared to the control group. Linear regressions within usage groups showed a correlation of high app usage and a reduction of psychological distress. Fatigue data presented a statistically significant antifatigue efficacy ( p < 0.001) and physical activity increased by 63.9%. Conclusion: App-based supportive care offers a promising, low-threshold, and cost-efficient opportunity to improve psychological well-being, quality of life, fatigue, and physical activity. More research is needed to implement eHealth solutions in clinical cancer care.