Close relationships are an important social context in which emotional experiences, regulation, and coregulation unfold. This interpersonal emotion regulation process is likely intertwined with the self-regulatory capacities and social skills of each individual dyad member. This study aimed to examine whether respiratory sinus arrhythmia (RSA), a physiological marker related to self-regulation, moderates the impact of rumination, a maladaptive emotion regulation strategy, on couples' conflict. A dyadic, longitudinal design examined the association among RSA, rumination, and couples' conflict in a sample of 83 cohabiting romantic partners raising young children. At baseline, rumination and RSA from each romantic partner were assessed. Couples' conflict was reported at 3 time points over the following 12 months. Actor-partner interdependence modeling examined the mutual contributions of each couple member's rumination to couples' conflict, as well as the moderating impact of RSA. Results indicated that rumination from both members of the dyad were independently associated with couples' conflict across the 12-month period. Furthermore, RSA moderated the association between one's partner's rumination and couples' conflict, such that high actor RSA attenuated the positive association between partner's rumination and couples' conflict. The findings highlight the interdependent nature of emotion regulation within close relationships, and the impact of RSA on interpersonal emotion regulation processes. (PsycINFO Database Record (c) 2019 APA, all rights reserved).