<p>Individual differences in respiratory sinus arrhythmia (RSA) are linked to emotion and behavior dysregulation, two transdiagnostic indicators of psychopathology youth. However, most research has focused on RSA reactivity to stressors, with little work examining individual differences in RSA <i>following</i> stressors. The current study examined associations between dynamic changes in RSA following parent–child conflict and youth emotion dysregulation and reactive aggression in daily life. In a transdiagnostic sample of clinically referred youth (<i>n</i> = 162; <i>M</i>_age = 12.03&#xa0;years, <i>SD</i> = 0.92; 47% female; 60% minoritized racial/ethnic status), RSA was assessed during a laboratory-based parent–child interaction task, which included a conflict discussion and four subsequent tasks. Emotion dysregulation and reactive aggression were assessed during a 4-day ecological momentary assessment protocol. Dynamic changes in RSA were modeled using a non-linear growth model (i.e., free curve slope intercept [FCSI] model), and associations with emotion dysregulation and reactive aggression were examined within a multivariate structural equation model. Results from the FCSI model demonstrated significant between-person differences (intercept) and within-individual change (slope) in RSA and predictive associations were specific to reactive aggression. Specifically, youth with higher mean levels of RSA (<i>β</i> = 0.20<i>, p</i> = .02<i>)</i> and those showing decreases in RSA (i.e., continued withdrawal) following parent–child conflict (<i>β</i> = -0.25<i>, p</i> = .03) were more likely to engage in reactive aggression in daily life. No associations wer found with emotion dysregulation. Findings underscore the importance of examining dynamic changes in RSA and suggest prolonged autonomic recovery following a conflict may be an indicator of risk for reactive aggression.</p>