OBJECTIVE: Exposure to trauma-related cues has been associated with a prolonged decrease in heart rate variability (HRV) under laboratory conditions, however the relationship between PTSD symptoms and HRV has not been evaluated during everyday life. The present study sought to determine whether Posttraumatic Stress Disorder (PTSD) symptoms reported during everyday life were related to reduced HRV. METHODOLOGY: Eighty-three young adults with PTSD underwent 24-hour Holter monitoring, during which PTSD symptoms were measured using ecological momentary assessment (EMA). Multilevel modeling was used to examine the association between PTSD symptom severity and low frequency (LF) and high frequency (HF) HRV. RESULTS: PTSD symptoms were associated with reductions in LF HRV, independently of age and activity level. There was no significant association between PTSD symptom levels and HF HRV. CONCLUSIONS: These results indicate that an association between momentary PTSD symptom severity and reduced LF HRV is significant and observable in young adults with PTSD. Findings highlight the need for cardiovascular screening in young adults with PTSD and early interventions that target physiological reactivity in PTSD.