Background: Befriending is an intervention intended to provide companionship and support to socially isolated populations. This review aimed to understand the key characteristics and psychological and social outcomes of befriending interventions for adults with intellectual disabilities. Methods: Systematic searches of electronic databases (PsycINFO, MedLine and Web of Science) identified 11 studies for inclusion. A narrative synthesis of the findings was completed, along with critical appraisal of study quality. Results: Increased community participation, positive changes to social networks and mood were frequently reported outcomes for befriendees. Increased knowledge, new experiences and opportunities to 'give back' were most reported for befrienders. Conclusions: The review highlighted that existing research in this field is limited in scope and methodologically diverse. Future research should focus upon the effectiveness and long-term impact of befriending interventions, understanding the mechanisms of change, and eliciting the views of people with intellectual disabilities on their experiences.