Regular physical activity (PA) reduces morbidity and mortality in prostate cancer. Prescribing PA in cancer is a necessary awareness but is a real challenge in the case of prostate cancer. Motivational peer support seems to be an innovative strategy for lifestyle change. Therefore, we developed the Acti-Pair programme and demonstrated its feasibility. We want to evaluate its effectiveness in promoting PA in patients with prostate cancer. The Acti-Pair 2 study is an interventional, comparative, multicentre, randomised, stepped-wedge cluster study. The control group will consist of patients being followed for prostate cancer and receiving advice and recommendations for PA during consultations to make patients more active in their daily lives (=usual practice, PA to be performed independently at home). The intervention group will consist of patients being followed up for prostate cancer and benefiting from the Acti-Pair programme, which combines three interventions: (1) motivational support from a peer; (2) construction of a personalised and realistic project and (3) support from health and adapted PA professionals. This study will assess the effectiveness, implementation and efficiency of the Acti-Pair programme. It will allow the identification of key success factors for implementing the Acti-Pair programme to prepare for its transferability. Trial registration number: Clinical trial, NCT05739565, registered on 20 February 2023, https://clinicaltrials.gov/study/NCT05739565..