Respiratory sinus arrhythmia (RSA) has been examined as a psychophysiological marker of stress vulnerability. Research indicates that low resting RSA is associated with physical and mental health problems, including posttraumatic stress disorder (PTSD). Some research suggests that people diagnosed with PTSD have lower RSA than people without PTSD, but findings have been mixed and the overall magnitude of this effect is unknown, indicating the need for a comprehensive meta-analysis. This meta-analysis examined the association between PTSD and baseline RSA in 55 studies, including 12 unpublished studies, with a total sample size of 6689. Studies were included if they used a PTSD measure, a baseline measure of RSA, and involved humans. Studies were excluded if they were not available in English, did not present quantitative data, presented duplicate data, were a case series, or did not provide results required for computing an effect size. The meta-analysis indicated there is a small but significant association between PTSD and RSA (g = -0.26; 95% CI = -0.35, -0.16) with moderate heterogeneity. Moderator analyses suggested that effects are larger for adults than for children and for DSM-5 PTSD measures than for non-DSM referenced measures. We found some evidence for publication bias among the meta-analysis findings. Overall, there is a small but reliable association between PTSD and lower resting RSA, providing support for further research examining the complex relationship between parasympathetic activity and PTSD.