Multiple recent studies have identified negative affective reactivity as a prospective predictor of physical and mental health outcomes. This study seeks to extend this line of research to relationship outcomes and specifically marital outcomes to see if individual differences in negative affective reactivity can predict these outcomes. Additionally, recent findings have shown links between negative affective reactivity (AR) and high-frequency heart rate variability (HF-HRV), an index of cardiac vagal control. An additional goal was to determine whether HF-HRV moderated the relationship between negative AR and marital quality. We used data from 344 participants who took part in waves II and III of the Midlife in the United States (MIDUS). Respondents completed daily diary measures of stress and affect, a psychophysiological procedure that included HF-HRV measures at rest, and survey scales on marital quality and covariates. We found that negative AR significantly predicted marital risk and marital satisfaction 10 years later and that this relationship was moderated by HF-HRV, which served as a stress buffer.