Biobehavioral frameworks of attachment posit that mother-child dyads engage in physiological synchrony that is uniquely formative for children's neurobiological, social, and emotional development. Much of the work on mother-child physiological synchrony has focused on respiratory sinus arrhythmia (RSA). However, the strength of the existing evidence for mother-child RSA synchrony during interaction is unclear. Using meta-analysis, we summarized results from 12 eligible studies comprising 14 samples and 1201 children ranging from infancy to adolescence (M[subscript age] = 5.68 years, SD = 4.13, range = 0.4-17 years) and their mothers. We found that there was a statistically significant, albeit modest, positive within-dyad association between mother and child fluctuations in RSA. There also was evidence for significant heterogeneity across studies. Less mother-child RSA synchrony was observed in high-risk samples characterized by clinical difficulties, history of maltreatment, or socioeconomic disadvantage. We did not find that mother-child RSA synchrony significantly differed by task context, mean child age, or by epoch length for computing RSA. Collectively, these findings suggest that mother-child dyads show correspondence in their fluctuations in RSA, and that RSA synchrony is disrupted in high-risk contexts. Future directions and implications for the study of parent-child physiological synchrony are discussed.