Despite supportive behaviors playing a central role in intimate relationships, the extent to which physiological and psychological factors are involved in the quality of the observed spousal support, remains largely unknown. From a physiological stance, cardiac synchrony has been identified as an important component involved in dyadic interpersonal interactions. This study aims to examine whether individual differences in attachment determine, at least to some extent, whether cardiac synchrony enhances or impedes the quality of the observed spousal support. Specifically, this study examines whether attachment style moderates the biobehavioral link between positive cardiac synchrony and observed spousal support. A total of 58 couples expecting their first child participated in a supportive interaction while their cardiovascular responses were recorded, and the quality of their caregiving behaviors was coded. Results indicated that couples' cardiovascular dynamics were synchronized during the supportive interaction. Furthermore, attachment moderated the association between positive cardiac synchrony and the behavioral manifestation of support, such that cardiac synchrony was negatively associated with the quality of support when offered by caregivers with higher scores of attachment anxiety. Our findings show that for those caregivers who have higher levels of anxious attachment—being synchronized with their care seeking spouses is associated with difficulties in providing effective support. We discuss these findings in the context of the emotion regulation difficulties characteristic of anxious attachment patterns. These findings suggest that future work could benefit from performing comprehensive studies that consider physiological, behavioral, and psychological constructs simultaneously.