Introduction: The occupational demands of firefighters are psychologically stressful and often traumatic, as firefighters respond to a wide range of emergencies. The stressful nature of firefighting results in increased susceptibility to PTSD and impaired heart rate variability (HRV) due to a continued stress response well after the disturbing situation has ended. High levels of psychological resilience may help offset a continued stress response, yet the associations between these variables have not been fully explored. Thus, the purpose of this study was to determine the relationship between HRV and psychological outcomes, including psychological resilience and PTSD severity, in full-time firefighters. Methods: Sixteen full-time male firefighters participated in this cross-sectional study. Participants completed the PTSD Checklist-5 (PCL-5) and Dispositional Resilience Scale-15 (DRS-15). Resting HRV was measured for 10 minutes with an actigraphy watch and chest heart rate monitor during nighttime sleep and then analyzed with Kubios HRV software to determine HRV outcomes. Partial correlation coefficient analyses were performed between (a) the PCL-5 score and (b) the DRS-15 score and each of the HRV measurements' mean RR interval, RMSSD, percentage of low-frequency power (LF), percentage of high-frequency power (HF), LF/HF ratio, parasympathetic nervous system index, and sympathetic nervous system index, after adjusting for age. SPSS version 29 was used for the statistical analysis (α = 0.05). Results: All correlation coefficients between the psychological scale variables and the HRV variables were found to be non-significant (p > 0.05 for all) and weak (-0.38 < r < 0.44). Conclusion: After adjusting for age, a relationship was not found between HRV and either PTSD severity or psychological resilience. However, the PCL-5 and DRS-15 scores were relatively homogeneous between participants, which may have contributed to the weak relationship found with HRV in this small sample size of firefighters. In further data analyses, we will continue to increase sample size and statistical power.