Inter- and intrapersonal affect (IIPA; mother-child warm emotional exchange and individual expressions of positive affect) supports adaptive child self-regulation in Black and Latinx families but has yet to be assessed in relation to youth's respiratory sinus arrhythmia (RSA). Contextual stressors (income, internalizing symptoms) can undermine Black and Latinx children's RSA regulation. Using a self-regulation promotion framework, this study examined associations between IIPA and children's RSA regulation using three distinct measurements, while controlling for contextual stressors. One hundred mothers (M = 34.48 years, SD = 6.39 years) and their children (M = 6.83 years, SD = 1.50 years) participated. Dyads completed a mildly stressful task where RSA and video recordings were captured continuously. Videos were later coded for IIPA. Mothers reported on income and internalizing symptoms. Relations between IIPA and the three child RSA regulation indices (i.e., suppression, instability, trajectory) were assessed. No significant relations between IIPA and RSA suppression or instability were found; however, IIPA was positively related to a gradual decrease in RSA and a slowing of RSA reactivity across the stress task in the trajectory model. Findings identify affective processes within Black and Latinx families that are advantageous for children's physiological regulation, even in the context of stress. Further, a trajectory model of youth's physiological regulation may allow for a more nuanced understanding of RSA. Collectively, findings underscore that IIPA may be advantageous for adaptive RSA regulation in Black and Latinx school-age youth in the context of stress.