Background. Fostering healthy urban living conditions is a critical public health objective. One efficient approach lies in the contact to nature, as numerous studies have shown that urban and peri-urban natural elements both indoors and outdoors carry a large potential in buffering typical urban threats to mental health. However, few studies have summarised these effects with a quantitative, meta-analytic approach. Methods. We employed a meta-analysis of quantitative (quasi)-experimental studies to synthesise findings on urban green and mental health. One hundred-and-seven pre-post control measurements within 56 experiments were summarised as the standardised mean difference within eight outcome groups, namely state anxiety, positive and negative mood, self-reported stress, depression, self-reported restoration, attention and working memory (i.e. the brain's system for briefly storing and processing information for immediate use). Main results. Significant and positive condensed small to medium effects were found for state anxiety (0.26, 95% CI [0.10; 0.42], based on k = 10 studies), self-reported stress (0.40, 95% CI [0.16; 0.64], k = 11), self-reported restoration (0.63, 95% CI [0.38; 0.88], k = 7), attention (0.47, 95% CI [0.27; 0.66], k = 5), depression (0.31, 95% CI [0.13; 0.50], k = 8), positive mood (0.41, 95% CI [0.27; 0.55], k = 31) and negative mood (0.22, 95% CI [0.07; 0.37], k = 26). This indicates that the nature intervention groups generally derived more pre-post benefits than the control groups. Non-significant results were found for working memory (0.22, 95% CI [−0.12; 0.56], k = 9). Exploratory subgroup analyses of self-reported stress and mood revealed differences by the type of nature contact and the sample characteristics (student majority vs. no student majority); however, most of these differences were non-significant. Secondary results. The included study samples were heavily student-based and globally skewed towards Europe and North America. Risk of bias was acceptable; however, the description of the study samples and the represented population were partially insufficient. Policy implications. Our results stress the importance of accessible and attractive urban green and encourage the strategic investment of public resources for greener cities. Read the free Plain Language Summary for this article on the Journal blog.