Abstract: Background: The coronavirus disease (COVID-19) pandemic led to the implementation of social distancing laws in the UK. This had several negative consequences on health, wellbeing and social functioning within the general population. Military veterans may have had unique experiences of social isolation during this time. This study examined the level of, and relationship between, structural and functional social support, and its association with mental health and wellbeing in a sample of UK veterans during the COVID-19 pandemic. Methods: Throughout the first summer of the pandemic (June-September 2020), an additional wave of cross-sectional data was collected from UK Armed Forces personnel who had left regular military service and were participating in a longitudinal cohort study. In total, 1562 participants (44.04% response rate) completed a series of online questionnaires measuring sociodemographic characteristics, COVID-19 experiences and psychosocial health and wellbeing. Multivariable logistic and ordinal regression analyses were conducted. Results: For structural social support, 86.76% were in a relationship and 88.96% lived with others. For functional social support, one-quarter reported feelings of loneliness (27.42%) and low levels of perceived social support (28.14%). Structural support was associated with functional support. Being single, living alone and experiencing loneliness were associated with worse mental health and wellbeing, while living with other adults and reporting high levels of perceived social support were associated with better mental health and wellbeing. Conclusions: This study has enhanced our understanding of social support among veterans and its implications for health and wellbeing. This knowledge is essential for advising the development of psychosocial interventions and policies to improve the psychological wellbeing of veterans in future pandemics and more broadly in their daily lives.