There is uncertainty as to how heart rate variability (HRV), an index of flexible physiological reactivity, relates to an individual’s tendency to experience negative emotions. Research suggests that high and low HRV may reflect maladaptive physiological processes associated with negative affectivity and that association may depend on an individual’s use of emotion regulation strategies, such as cognitive reappraisal. The current study examined the moderating role of cognitive reappraisal in the quadratic association between HRV and negative affectivity. Electrocardiograms were recorded at rest for 269 young adults who then completed self-report rating scales assessing trait negative affectivity and trait cognitive reappraisal. As predicted, high and low HRV were associated with high negative affectivity at low levels of cognitive reappraisal. These results suggest that, contrary to traditional views, high HRV may not be an adaptive characteristic in the context of low trait reappraisal.