BACKGROUND: Reduced mobility is the first sign of functional decline and can lead to dependency in elderly people. Screening for the risk of mobility limitation in this population is an important public health issue to prevent further disabilities. Despite the current lack of guidelines, primary care healthcare providers may have a central role to play in this type of screening. Multi-domain physical exercise interventions in older persons have shown some efficacy/effectiveness on frailty status, yet, to the best of our knowledge, no published study has focused on patients screened in primary care. METHOD: The PRISME-3P study is a national, interventional, multicenter, cluster randomized trial. Patients over 70 years of age will be systematically screened by their general practitioner (GP) on the basis of clinical criteria of mobility limitation. To avoid contamination bias, the unit of randomization will be the GP practice. In the intervention group, patients will consult a geriatrician and a dietician, and will receive a physical training program from a personal trainer who will demonstrate the exercises and provide follow-up coaching. The control group will receive standard care. The primary outcome will be the change in Short Physical Performance Battery (SPPB) scores between inclusion and 6-months follow-up. DISCUSSION: We expect an improvement of the SPPB between inclusion and 6 months of follow-up. TRIAL REGISTRATION: This study is registered in ClinicalTrials.gov ( NCT02847871 , 27 July 2016).