Background: A majority of nursing home residents do not fulfill physical activity guidelines. Many existing interventions to promote physical activity and prevent falls in nursing home residents have low adherence rates, lack theoretical foundation, or require a large amount of time from staff for preparation and/ or delivery. Objective: To describe the rationale, development steps, and implementation approach of the BeStAge app through a cluster randomized controlled trial (cRCT), as well as to provide baseline characteristics of the study sample and discuss possible implications for further app developments. Methods: We iteratively developed a novel, tablet-based mobile application named BeSt Age app that enables nursing home staff to deliver individualized physical exercise training to residents with or without motor or/ and cognitive impairments. The app was designed and developed based on an intervention-mapping approach. A needs assessment was performed, followed by defining objectives, theory-based methods, program development, implementation, and evaluation. We took several steps to ensure that the app was based on a sound theoretical background, as well as results derived from and limitations identified in prior empirical research. For implementation and evaluation purposes, we conducted a cRCT, and results from the cRCT will be used to examine the effectiveness of the app with regard to different outcomes of interest. Primary outcomes among participating nursing home residents are quality of life, fall risk evaluated through two performance-oriented balance tests, and fall incidence. Secondary outcomes include motor performance, cognition, activities of daily living, life space/ physical activity behavior, and fall efficacy, amongst others. Results: 229 older adults from 19 different nursing homes (171 females, 58 males; mean age 85 years) participated in the cRCT. The intervention group (IG; N = 137 from 11 nursing homes) received a 12-week intervention with the BeSt Age app in small, homogenous groups of 5-7 nursing home residents, with two exercise sessions per week, each lasting 25-30 minutes. The control group (CG; N = 92 from 8 nursing homes) received usual care. At baseline, the IG had a statistically significantly larger number of females, participants had a higher body mass index, and more participants rated attending physical activity programs as important; there were no further statistically significant differences between IG and CG. Results with regard to the effectiveness of the BeSt Age app are expected to be published in summer/ fall of 2025. Conclusions: If proven effective, the BeSt Age app may be a viable solution for physical activity promotion and fall prevention among older adults residing in nursing homes, thereby ultimately contributing to maintaining quality of life and overall well-being in this vulnerable population. The app can support nursing home staff in delivering exercise training to residents with minimal additional workload and without requiring specific resources. Clinical Trial: German National Register of Clinical Trials (DRKS00032349)