Social isolation and loneliness are frequently associated with heart failure. It is unclear how these constructs are assessed in adults living with heart failure which warrants further exploration. This review aimed to identify how social isolation and loneliness is assessed in adults living with heart failure. This is a systematic review reported according to PRISMA and registered in Prospero on 18 March 2024 [CRD42024518571]. The bibliographic databases, MEDLINE, CINAHL, and Scopus were searched from inception to 20 March 2024. Original quantitative studies assessing loneliness and/or social isolation of adults living with heart failure using a patient-reported instrument and written in English language were included. The Joanna Briggs Institute Critical Appraisal checklists were used to assess the quality of included studies. The results were presented narratively. Thirty studies (17 cohort studies, 9 cross-sectional studies, 2 RCTs, and 2 case control) with 529,665 participants (mean age ranged from 52 to 83 years, 57% were women) were included. The University of California Los Angeles Loneliness Scale was the most commonly used instrument to assess loneliness while composite measures of network size and frequency of social contacts were the most commonly used to assess social isolation in adults living with heart failure. Social isolation and loneliness exert deleterious effects on both mental and physical health, significantly diminishing life satisfaction. The improved use of social isolation and loneliness assessment instruments may contribute to more effective interventions, ultimately enabling care that may enhance the health outcomes and quality of life of adults living with heart failure.