Contact with nature is widely considered to ameliorate psychological stress, but the empirical support for a causal link is limited. We conducted a systematic review to synthesize and critically assess the evidence. Six electronic databases were searched. Twenty-six studies evaluated the difference between the effect of natural environments and that of a suitable control on the acute psychophysiological stress response. Eighteen studies were rated as being of moderate quality, four studies of low quality, and four studies of high quality. Meta-analyses indicated that seated relaxation (g = .5, p = .06) and walking (g = .3, p = .02) in natural environments enhanced heart rate variability more than the same activities in control conditions. Cortisol concentration measures were inconsistent. While intuitively and theoretically sound, the empirical support for acute stress-reducing effects of immersion in natural environments is tentative due to small sample sizes and methodological weaknesses in the studies. We provide guidelines for future research.