Family caregivers of people with severe mental illness (SMI) have been increasingly observed to experience social isolation and/or loneliness (SI/L) which are risk factors for ill health. This scoping review aimed to map existing evidence and identify knowledge gaps in studies on SI/L in this population using the Arksey and O'Malley's framework. Parallel searches (2011–2021) conducted in 10 databases identified 51 publications from 18 countries fully meeting the inclusion criteria. Over half of the included studies were quantitative. We found that the definition of loneliness reached a consensus, while the definition of social isolation varied across studies. Risk factors and correlates of SI/L were grouped into sociodemographic factors, illness‐related factors, health and wellbeing, and stigma. The evidence showed a lack of comprehensive measurements assessing SI/L, few longitudinal studies, and little knowledge of interventions specifically addressing SI/L. Future studies are recommended to address these knowledge gaps and explore effective interventions on SI/L in family caregivers of people with SMI.