This study investigated whether parents and kindergarten children show concurrent and time-lagged physiological synchrony during dyadic interaction. Further, we tested whether parent–child behavioral co-regulation was associated with concurrent and time-lagged synchrony, and whether synchrony varied by the type of interaction task. Participants were 94 children (Mage = 5.6 years, 56% female) and their parents. We simultaneously measured parent and child respiratory sinus arrhythmia (RSA) during four dyadic interaction tasks: free play, clean up, problem-solving, and puzzle teaching. We found that synchrony varied by task. Concurrent synchrony occurred only during the puzzle teaching task, such that parent and child RSA were significantly and positively associated with each other simultaneously. Time-lagged synchrony occurred only during the problem-solving task, such that parent RSA was positively associated with child RSA 30 seconds later, and child RSA was negatively associated with parent RSA 30 seconds later. Although behavioral co-regulation and physiological synchrony have been conceptualized as markers of responsive parent–child interactions, our study finds no evidence that physiological synchrony is associated with between-dyad differences in behavioral co-regulation.