The aim of this study is to assess whether positive emotional exchanges (i.e., emotion coregulation) within the mother–child dyad play a protective role in children's physiological response to a distressing task. Specifically, we test whether positive emotion coregulation among mothers and their preschool-aged children is associated with children's respiratory sinus arrhythmia (RSA) at baseline, during, and following a frustration task. One hundred Singaporean mother–child dyads (Mchildage = 3.5 years) participated in a standardized “Laughing Task” in which positive emotional constructs were measured. Children also participated in a frustration task while RSA was continuously monitored. Hierarchical linear regressions revealed that greater maternal positive emotional responses to children were associated with child RSA at baseline and in recovery from frustration, but not during frustration. These findings have implications for the important role that positive emotion responsivity from mothers may play in children's developing autonomic response systems, and underscore the need for longitudinal work on this topic.