PurposeThis is a study protocol to co-create with knowledge users a core outcome set focused on middle-aged and older adults (40 years+) for use in social prescribing research.MethodsWe will follow the Core Outcome Measures in Effectiveness Trials (COMET) guide and use modified Delphi methods, including collating outcomes reported in social prescribing publications, online surveys, and discussion with our team to finalize the core outcome set. We intentionally center this work on people who deliver and receive social prescribing and include methods to evaluate collaboration. Our three-part process includes: (1) identifying published systematic reviews on social prescribing for adults to extract reported outcomes; and (2) up to three rounds of online surveys to rate the importance of outcomes for social prescribing. For this part, we will invite people (n = 240) who represent the population experienced in social prescribing, including researchers, members of social prescribing organizations, and people who receive social prescribing and their caregivers. Finally, we will (3) convene a virtual team meeting to discuss and rank the findings and finalize the core outcome set and our knowledge mobilization plan.ConclusionTo our knowledge, this is the first study designed to use a modified Delphi method to co-create core outcomes for social prescribing. Development of a core outcome set contributes to improved knowledge synthesis via consistency in measures and terminology. We aim to develop guidance for future research, and specifically on the use of core outcomes for social prescribing at the person/patient, provider, program, and societal-level.