Several decades of research have demonstrated that marital relationships have a powerful influence on physical health. However, surprisingly little is known about how marriage affects healthboth in terms of psychological processes and biological ones. Over a 10-year period, we investigated the associations between perceived partner responsivenessthe extent to which people feel understood, cared for, and appreciated by their romantic partnersand diurnal cortisol in a large sample of married and cohabitating couples in the United States. Partner responsiveness predicted higher cortisol values at awakening and steeper (i.e., healthier) cortisol slopes at the 10-year follow-up. These associations remained strong after we controlled for demographic factors, depressive symptoms, agreeableness, and other positive and negative relationship factors. Furthermore, declines in negative affect over the 10-year period mediated the prospective association between responsiveness and cortisol slope. These findings suggest that diurnal cortisol may be a key biological pathway through which social relationships affect long-term health.