Physiological synchrony is an important biological process during which parent-child interaction plays a significant role in shaping child socioemotional adjustment. The present study held a context-dependent perspective to examine the conditional association between parent-child physiological synchrony and child socioemotional adjustment (i.e., relationship quality with parents and child emotion regulation) under different (i.e., from highly unsupportive to highly supportive) emotional contexts. One hundred and fifty school-age Chinese children (M[subscript age] = 8.64 years, 63 girls) and their primary caregivers participated in this study. After attaching electrocardiogram (ECG) electrodes, parent-child dyads were instructed to complete a 4-minute conflict discussion task. Parent-child physiological synchrony was calculated based on the within-dyad association between parents' and children's respiratory sinus arrhythmia (RSA) levels across eight 30-second epochs. Parental emotional support, child relationship quality with parents, and child emotion regulation during the discussion task were coded by trained research assistants. Supporting our hypotheses, parental emotional support moderated the relations of parent-child RSA synchrony with both child relationship quality with parents and child emotion regulation. Furthermore, the Johnson-Neyman technique of moderation indicated that the associations between parent and child RSA synchrony and child socioemotional adjustment indicators shifted from negative to positive as the parental emotional support became increasingly high. Our findings suggest that parent-child physiological synchrony may not be inherently adaptive or maladaptive, highlighting the importance of understanding the function of parent-child physiological synchrony under specific contexts.