Objective: This cross-sectional study examines associations between cardiac vagal control and mainstream acculturation by systematically investigating the independent contributions of resting, reactivity, and recovery Respiratory Sinus Arrhythmia (RSA)—physiological factors contributing to adaptive functioning, including social engagement capacity. This study also examines the moderating role of RSA reactivity in associations between perceived ethnic discrimination and mainstream acculturation. Method: The sample comprised 111 migrants from Maghreb (Morocco, Algeria, and Tunisia) to Montreal, QC, Canada. On average, participants (60 women) were 31 years old and had immigrated 7 years prior to the study. The study design was cross-sectional. We collected both psychophysiological (electrocardiogram) and self-report questionnaire data. Results: All three facets of cardiac vagal control are independently associated with mainstream acculturation: Higher mainstream acculturation was associated with higher resting RSA (β = .24), lower RSA reactivity (β = −.27), and higher recovery RSA (β = .26), controlling for several theoretically relevant covariates. Furthermore, lower RSA reactivity to a discrimination-recall task dampened the relation between perceived discrimination and mainstream acculturation (interaction β = −.20). Conclusions: This study replicates and extends prior work linking RSA and mainstream acculturation using a sample of racialized and marginalized migrants. It provides evidence that RSA is related to migrants’ adoption of the mainstream culture and may modulate how they handle perceived ethnic discrimination. Thus, this study also provides evidence that RSA is tied to how individuals successfully navigate novel social environments, including novel cultural contexts.