While interpersonal synchrony is regarded as a defining characteristic of early social exchanges between infants and their caregivers, the mechanisms facilitating synchronous interactions are poorly understood. The goal of the present study was to examine the relationship between maternal playful singing and interpersonal synchrony of the mother-infant dyad. Overall, 56 mothers and their 4-month-old infants were observed during natural interactions that were then micro-coded for maternal and infant social behaviours as well as maternal singing and rhyming in the context of social game routines. Results showed that 38 dyads spontaneously engaged in social game routines. In these dyads, both playful singing and rhyming were positively associated with dyadic gaze synchrony, while only playful singing was also positively correlated with affect synchrony of the dyad. These findings suggest that rhythms, in general, may have important implications for the establishment of interpersonal synchrony in infant-caregiver dyads. However, musical rhythms seem to be particularly emotionally-salient and thus attune both interactional partners to the affective content of their social exchanges.