To design and develop a complex, evidence‑based, theory‑driven, and culturally appropriate character strengths-based intervention (CSI) for breast cancer patients, following the Medical Research Council (MRC) framework. From 2018 September to 2020 November, a complex intervention perspective was adopted. The rationale, methods, and processes employed in carrying out the study were reported. The acceptability and feasibility of intervention program were evaluated as a part of subsequent pilot study. Based on piloting, a refined and optimized definitive intervention was obtained. The development of the intervention is an iterative process involving input from three key stakeholders: experts, medical staff, and patient representatives. The systematic review revealed CSIs were effective and the selected theory served as a guide and indicated theory-inspired modifications. A representative team of breast cancer patients and oncology nurses collaboratively developed and tailored the intervention content and format with attention to the acceptability and feasibility. Five main strategies, including peripheral, evidential, linguistic, constituent-involving, and sociocultural strategies, were used to achieve and strengthen the cultural appropriateness. After the pilot phase, several refinements were made on the CSI program, such as editorial changes in the booklet or alternative suggestions for difficult strengths-based activities (e.g., outdoor activities). All participants not only expressed satisfaction with the program in process evaluation, but also reported perceived benefits such as enjoyable and sociable experience, better well-being, and increased confidence. Consideration of the MRC framework, theory guidance, and suggestions from stakeholders during intervention development can optimize uptake and sustainability in the clinical setting. It is recommended that randomized controlled trial be used in future studies to assess the intervention, the process and the mechanisms of the intervention. Our approach may offer implications for the design and implementation of similar initiatives to support cancer patients. ClinicalTrials.gov Register Identifier: NCT04219267, 07/01/2020, retrospectively registered.