The polyvagal theory (Porges, 2007) represents a biobehavioral model that relates autonomic functioning to self-regulation and social engagement. The aim of the two presented studies was to test the proposed association of cardiac vagal tone (CVT), assessed via resting high-frequency heart rate variability (respiratory sinus arrhythmia, RSA), with coping, emotion-regulation, and social engagement in young adults. In Study 1 (retrospective self-report), RSA was positively associated with engagement coping (situation control, response control, positive self-instructions, social-support seeking) and aspects of social well-being. In Study 2 (ecological momentary assessment), for 28 days following the initial assessment, RSA predicted less use of disengagement strategies (acceptance and avoidance) for regulating negative emotions and more use of socially adaptive emotion-regulation strategies (i.e., social-support seeking as a reaction to sadness and making a concession as a reaction to anger caused by others). Furthermore, RSA was higher in participants who reported no anger episodes compared to those who reported at least one anger episode and was positively associated with reported episodes of negative emotions. Results support the association proposed by the PVT between CVT and self-regulatory behavior, which promotes social bonds. Copyright © 2013 Elsevier B.V. All rights reserved.