Abstract Background Social relationships play a fundamental role in individuals’ lives and health, and social isolation is prevalent among older people. Chronic non-communicable diseases (NCDs) and frailty are also common in older adults. Aims To examine the association between number of NCDs and social isolation in a cohort of community-dwelling older adults in the UK, and to consider whether any potential association is mediated by frailty. Methods NCDs were self-reported by 176 older community-dwelling UK adults via questionnaire. Social isolation was assessed using the six-item Lubben Social Network Scale. Frailty was assessed by the Fried phenotype of physical frailty. Results The median (IQR) age of participants in this study was 83.1 (81.5–85.5) years for men and 83.8 (81.5–85.9) years for women. The proportion of socially isolated individuals was 19% in men and 20% in women. More women (18%) than men (13%) were identified as frail. The number of NCDs was associated with higher odds of being isolated in women (unadjusted odds ratio per additional NCD: 1.65, 95% CI 1.08, 2.52, p = 0.021), but not in men, and the association remained robust to adjustment, even when accounting for frailty (OR 1.85, 95% CI 1.06, 3.22, p = 0.031). Discussion Number of self-reported NCDs was associated with higher odds of social isolation in women but not in men, and the association remained after considering frailty status. Conclusions Our observations may be considered by healthcare professionals caring for community-dwelling older adults with multiple NCDs, where enquiring about social isolation as part of a comprehensive assessment may be important.