Considerable heterogeneity has been observed in couples’ adjustment to the transition to parenthood (TTP). One potential yet understudied predictor of emotional adjustment to the TTP is the new parents’ capacity for regulation. A widely accepted biological marker of this capacity is respiratory sinus arrhythmia (RSA), which is closely tied to parasympathetic activation. In the present work, we sought to examine the role of tonic RSA and RSA reactivity as possible protective dyadic factors in the TTP. As part of a larger study, we recruited a sample (N = 100) of TTP couples. At 15 weeks postpartum, the couples took part in a lab session during which their RSA was assessed both at rest (tonic RSA) and during four affiliative dyadic interactions (RSA reactivity). Following this session, couples completed daily diaries over a period of 3 weeks, reporting their daily levels of negative affect and stress. A Multivariate Actor Partner Interdependence Model was used to examine the extent to which each partner's RSA predicted their own and their partner's negative affect (NA) level, as well as NA stress‐reactivity (i.e., the strength of the within‐person stress‐affect association). New mothers’ tonic RSA predicted their own lower NA level and NA stress‐reactivity; both their tonic RSA and RSA reactivity predicted their (male) partners’ lower NA level; and finally, new fathers’ tonic RSA and RSA reactivity predicted their (female) partners’ lower NA stress‐reactivity. These results suggest that RSA may serve as a personal and dyadic protective factor.