Significance: The planned RCT will test the efficacy of the adapted German PRO-SELF© Plus PCP, with findings from the qualitative substudy providing additional insights. Qualitative results will support the interpretation of quantitative results and vice versa. If efficacious in decreasing average and worst pain in patients with cancer by improving patients' and FCs' pain self-management, the adapted German PRO-SELF© Plus PCP could be adapted to and implemented in clinical practice. Specially trained oncology nurses in outpatient clinics and home care organizations could apply the intervention as needed.