Research has shown that the quality of mother-infant interactions, as measured by mother-infant synchrony, is associated with infants' vagal tone, a physiological indicator of emotion regulation. However, little is known about the association between the infant's vagal tone and the quality of father-infant interactions. Existing literature suggests that the greater quantity of time a father spends with the infant, either alone (dyadic time) or with others (social time) may strengthen this association. In this study we investigated the association between father-infant synchrony and infants' vagal tone in 77 dyads made up of fathers and their 3-month-old infants, considering the moderating roles of dyadic time and social time. Results revealed no direct associations between father-infant synchrony and infants' vagal tone. Moderation analyses revealed that more social time increases the association between synchrony and vagal tone. This result indicates a potential influence of family interactive processes.