OBJECTIVE: This ecological study explored the association between regulatory emotional self-efficacy beliefs in managing negative emotions (RESE-NE) and heart rate variability (HRV), a measure of parasympathetic modulation of the heart that has been positively associated with a better ability to flexibly adjust to a changing environment and regulate emotions. METHOD: To test these associations, we used data from 161 adults working in relational professions (about 40% men; mean [standard deviation] age = 40.45 [14.17] years) whose HRV was assessed continuously for 24 hours. RESULTS: Individuals high in RESE-NE showed increased HRV levels ( beta = -0.05, p = .011). However, this effect was moderated by biological sex ( beta = -0.18, p = .0001) such that RESE-NE positively and significantly predicted HRV for men ( beta = 0.18, p < .0001) but not for women ( beta = -0.001, p = .989). CONCLUSIONS: All in all, our study provides initial empirical support for the theoretical expectation derived from social cognitive theory that RESE-NE is positively correlated with a physiological indicator of adaptability to the environmental demands. This was true only for men, likely because of sex differences in physiological stress reactivity and regulation.