Both parasympathetic nervous system regulation and receipt of social support from close relationships contribute to prosocial development, although few studies have examined their combined influences in adolescence and particularly within racially and ethnically minoritized populations. In this longitudinal study of 229 U.S. Mexican-origin adolescents (48% female-identifying), youths reported on receipt of social support from family and friends from 10 to 16 years, had their baseline respiratory sinus arrhythmia (RSA) measured at 17 years, reported their prosocial behavior and completed the Mind in the Eyes test to assess cognitive empathy at 17 and 19 years, and reported their prosocial civic behavior (i.e., community activity) at 19 years. Family social support predicted prosocial behavior at 17 years, and friend social support predicted prosocial civic behavior at 19 years. Compared to youths with lower or higher baseline RSA, youths with moderate RSA reported more prosocial civic behavior, had greater cognitive empathy, and tended to report more general prosocial behavior at 19 years. The quadratic association between baseline RSA and cognitive empathy was stronger for youths with greater family social support. These findings are the first to extend the evidence that moderate baseline parasympathetic nervous system activity supports prosocial development into late adolescence and with the U.S. Mexican-origin community, and these findings address calls for more integrative biopsychosocial studies of prosociality.