Veterans with posttraumatic stress symptoms exhibit reduced heart rate variability characteristic of autonomic nervous system dysregulation. Studies show heart rate variability biofeedback (HRVB) is effective in reducing posttraumatic stress symptoms by improving autonomic functioning. Participants in this pilot study were veterans of different war eras with military-related posttraumatic stress symptoms. The study aims were to examine the impact of a single session HRVB intervention on posttraumatic stress symptoms and heart rate variability, test persistence of effects, and determine if veterans would find the intervention acceptable. One group (n = 6) received training in diaphragmatic breathing and heart rate variability biofeedback, augmented by twice-daily practice using a smart phone and breath pacing app. A second group (n = 6) received only a single session of diaphragmatic breathing training. After 4 weeks, participants in the second group (n = 5) received the full intervention. HRVB significantly reduced global posttraumatic stress symptoms, whereas diaphragmatic breathing alone did not. Further, veterans found the approach acceptable, as demonstrated by a high degree of adherence with prescribed practice, low study attrition, and continued use over time. Results of this pilot study warrant further refinement of a protocol utilizing mHealth to treat posttraumatic stress symptoms in military populations.