Adaptive affective responses in the face of environmental challenges require flexible physiological responding. The present study examined the extent to which tonic respiratory sinus arrhythmia (RSA)—a putative marker of regulatory capacity—moderated the association between stress-related changes in RSA (ie, phasic RSA) and concurrent changes in affect. Ninety-eight healthy, young adults completed ratings of affect during a resting baseline and following the recall of a recent stressor. Tonic RSA moderated the association of phasic RSA with stress-related change in positive affect (PA), such that change in RSA had a positive association with PA for individuals with higher tonic RSA and a negative association for those with lower tonic RSA. Examination of specific aspects of PA indicated that phasic RSA was positively associated with changes in ratings of attentive engagement among individuals with higher tonic RSA. These findings inform our understanding of phasic RSA and support the notion that flexible parasympathetic nervous system functioning is an important component of adaptive stress regulation.