Individuals with type 2 diabetes must engage in daily self-management behaviors to prevent complications. Given that management may be shared with a person's romantic partner, we examined both patients' and their partners' perceptions of relationship characteristics that were hypothesized to affect patients' self-efficacy for diabetes management. Adults with type 2 diabetes and their partners (n = 52 couples, 104 individuals) completed measures of three aspects of relationships that are theorized to affect self-efficacy: partner investment, partner support, and relationship satisfaction. Patients reported their self-efficacy for diabetes management and weekly frequency of diabetes self-management behaviors. A common fate modeling approach in which constructs were modeled as agreement between partner reports showed that relationship factors (investment, support, and satisfaction) significantly predicted patient self-efficacy (R 2 = 0.49), which in turn predicted patient self-management behaviors. This model fit the data well [χ2 (41) = 48.60, P = 0.19; comparative fit index (CFI) = 0.96; root mean square error of approximation = 0.06; and standardized root mean square residual = 0.07]. Interventions designed to support patients in their self-efficacy for self-management behavior may be improved through consideration of patients' romantic relationships.