Diabetes mellitus is a chronic disease with significant morbidity and mortality and it is associated with poor cognitive performance in later life. This study seeks to determine the relationship between social support and cognitive function among participants with type 2 diabetes mellitus (T2DM). We used data from the Reasons for Geographic and Racial Differences in Stroke (REGARDS) study, including participants with T2DM aged 45 and older (n = 4821). We examined different aspects of perceived social support, measured as structural social support (e.g., marital status), functional social support (having a caregiver in case of sickness or disability), and loneliness. We examined cognitive functioning using a six-item screener. Our results indicate that adults who felt lonely for 5-7 days per week had almost double the odds of cognitive impairment compared to those who didn't feel lonely. These results suggest that among middle-aged and older individuals with T2DM, interventions targeting lonely adults and which aim to reduce loneliness may combat some of the risks of cognitive decline.