OBJECTIVE To examine the association between marital interaction quality during daily life and subclinical cardiovascular disease (CVD). Studies have shown that marital status and quality of marriage are associated with cardiovascular health. However, little is known about the role of marital interaction quality during daily life in contributing to these effects. METHODS The sample consisted of 281 healthy, employed middle-aged adults who were married or living with a partner in a marital-like relationship (mean age = 42.0 years, 88% white, 52% men). Marital interaction quality was assessed using hourly real-time ecological momentary assessments for 4 days, with participants rating their current or recent partner interactions on positive and negative characteristics (e.g., agreeableness and conflict). Carotid artery intima-medial thickness (IMT) was assessed using ultrasound imaging. RESULTS Adjusting for demographics, positive marital interaction was inversely associated with IMT (b = -0.02, F(1,275) = 9.18, p = .002), and negative marital interaction was positively associated with IMT (b = 0.02 F(1,275) = 10.29, p = .001). These associations were not accounted for by behavioral and biological CVD risk factors and were consistent across age, sex, race, and education. The associations were also independent of marital interaction frequency, nonmarital social interaction quality, and personality factors. Global reports of marital quality, in contrast, were not associated with IMT. CONCLUSIONS Marital quality as measured during real-time interactions between partners was associated with subclinical CVD in healthy middle-aged adults. This study supports the use of real-time social interaction assessment for characterizing links between social relationships and cardiovascular health.