Background Special care units are a well-utilized approach in the long-term care for persons with dementia. A therapeutic goal of such settings is to provide meaningful engagement and a sense of community that is crucial for the overall quality of life. In recent years, several studies followed this notion by investigating residents’ social interactions and the influence of the environment on these interactions. Aims This review aims to synthesize the literature on the social interactions of persons with dementia living in special care units. Design A mixed-methods systematic review was conducted. Methods Literature was searched in PubMed, CINAHL, PsycINFO, the Cochrane Library and Web of Science databases. Additionally, reference lists of relevant articles were searched. Studies were screened, data were extracted and the quality was appraised. Separate syntheses were conducted for qualitative and quantitative studies, which were subsequently merged in the final mixed-methods synthesis. Results In total, 18 articles were included, investigating large-scale, small-scale and homelike special care units and green care farms. Residents in special care units experience few social interactions but more than those in the comparative groups. Opportunities to interact are only marginally seized. Interactions typically occur in small groups and are facilitated by familiarity and the organizational environment. Residents mainly rely on staff members to create social interaction, for example initiating or facilitating resident-to-resident interaction. Conclusion Although the evidence base is increasing, it is still fragmented and built on different concepts, interventions, control groups and measurements. Nevertheless, the first conclusions suggest a positive impact of special care units on residents’ social interactions. Although the review yielded a more comprehensive picture of residents’ social life, further high-quality research built on a sound theoretical background is needed.