Background: A number of studies have associated natural outdoor environments with reduced mortality but there is no systematic review synthesizing the evidence. Objectives: We aimed to systematically review the available evidence on the association between long-term exposure to residential green and blue spaces and mortality in adults, and make recommendations for further research. As a secondary aim, we also conducted meta-analyses to explore the magnitude of and heterogeneity in the risk estimates. Methods: Following the PRISMA statement guidelines for reporting systematic reviews and meta-analysis, two independent reviewers searched studies using keywords related to natural outdoor environments and mortality. Discussion: Our review identified twelve eligible studies conducted in North America, Europe, and Oceania with study populations ranging from 1645 up to more than 43 million individuals. These studies are heterogeneous in design, study population, green space assessment and covariate data. We found that the majority of studies show a reduction of the risk of cardiovascular disease (CVD) mortality in areas with higher residential greenness. Evidence of a reduction of all-cause mortality is more limited, and no benefits of residential greenness on lung cancer mortality are observed. There were no studies on blue spaces. Conclusions: This review supports the hypothesis that living in areas with higher amounts of green spaces reduces mortality, mainly CVD. Further studies such as cohort studies with more and better covariate data, improved green space assessment and accounting well for socioeconomic status are needed to provide further and more complete evidence, as well as studies evaluating the benefits of blue spaces.