Objective To examine whether individual differences in Type D personality (combination of negative affectivity (NA) and social inhibition (SI)) could explain heterogeneity in perceived social support and relationship adjustment (intimate partner relationship) among people living with diabetes. Design In the Diabetes MILES—The Netherlands survey, 621 adults with type 1 or type 2 diabetes (54% female, age: 56 ± 14 years) completed measures of Type D personality (DS14), perceived social support and relationship adjustment. We used established DS14 cut-off scores to indicate Type D personality, high NA only, high SI only and reference groups. Results Participants from the Type D and NA only groups perceived lower levels of social support (Welch[3,259] = 37.27, p < 0.001), and relationship adjustment (Welch[3,191] = 14.74; p < 0.01) than those from the SI only and reference groups. Type D was associated with lower social support (lowest quartile; adjusted OR = 8.73; 95%CI = 5.05 ∼ 15.09; p < 0.001) and lower relationship adjustment (lowest quartile; adjusted OR = 3.70; 95%CI = 2.10 ∼ 6.53; p < 0.001). Type D was also associated with increased levels of loneliness. Conclusion Participants with Type D and participants with high NA only tend to experience less social support and less relationship adjustment. Type D personality was also associated with more loneliness. Experiencing lower social support and relationship adjustment may complicate coping and self-management in people with diabetes.