The effects of power posing on hormonal reactions such as testosterone and cortisol have been widely investigated, however, its effects on the autonomic nervous system are rather unknown. Consequently, the aim of this study was to investigate the influence of power posing on cardiac vagal activity (CVA), as indexed by heart rate variability. It was hypothesized that high power poses (HPP) would increase CVA, whereas low power poses (LPP) would decrease CVA, given power posing is expected to decrease stress. Participants (N = 56) performed a total of four power poses, a combination of two power conditions (high vs. low) and two body positions (sitting vs. standing) for 1 min each, in a randomized order. In addition, for each power pose participants were given a role description. Contrary to our hypothesis, CVA decreased significantly during HPP in comparison to the resting measures before and after HPP, and CVA did not change during LPP. Moreover, while holding the power pose, CVA was higher in the LPP than in the HPP condition. Regarding subjective measures our hypotheses were confirmed, felt power was significantly higher after HPP than after LPP. Additionally, perceived stress was higher after LPP than after HPP. Taken together, these results suggest that the immediate impact of PP on the autonomic nervous system is more likely to influence a higher state of activation within the body instead of increasing resources to cope with stress as indexed by CVA, which may be seen only on a more long-term basis.