Abstract Using functional near-infrared spectroscopy (fNIRS) hyperscanning methodology, this study investigated whether parent emotional support moderated the relation between parent–child interbrain synchrony and interaction quality (via behavioral observation and child-report), controlling for individual emotional distress. Eighty-eight parent–child dyads (96.6% Han ethnicity), including a school-age child between the ages of 6 and 11 (M age = 8.07 years, SD = 1.16 years; 58.0% boys) and their parent (M age = 39.03 years, SD = 3.54 years; 69.3% mothers), participated in a cooperative task during which brain activity was assessed. Cluster-based permutations indicated parent–child interbrain synchrony in the left and right temporoparietal junction (TPJ). Interbrain synchrony in the left TPJ positively related to parent–child interaction quality in the context of high parent emotional support, whereas the association was weaker and negative when parents demonstrated low emotional support. Findings suggest the emotional context of an interaction is critical when assessing interbrain synchrony.